This issue had a long history in England, so the context and reasoning behind the 2nd amendment, which have generated such wildly different interpretations today, were 100% clear to contemporaries. The colonists considered themselves to be Englishmen and Englishwomen, and they instituted laws and customs that were closely based on those of England.
England was unlike the Continent in that it had a tradition of militias. This went back to the days of long bows, when each village was required to have a target and people had to practice. Service in the militia was compulsory for males age 16-60. They kept their arms at home and were expected to use them to defend their own homes. There was also an obligation to raise a hue and cry when violence happened, to maintain watch and ward at town gates, and to pursue criminals, even into other counties.
Militia service was compulsory. The alternative to a compulsory and universal militia would have been a volunteer force. A volunteer force, called a "select militia," was considered politically bad, because then you'd have an armed minority that could exercise power or be used as a tool. A standing army was even worse. England had a history of militias successfully resisting government oppression. At one point the king tried to get a royal monopoly on gunpowder, but he was successfully resisted.
English political theorists saw the right to bear arms as a necessary way of preserving one's own freedom, as opposed to "parchment rights." For example, Blackstone (1723-1780) writes:
[The constitution] has therefore established certain other auxiliary subordinate rights of the subject, which serve principally as outworks or barriers to protect and maintain inviolate the three great and primary rights, of personal security, personal liberty, and private property: [auxiliary rights 1-4 are listed, and then] 5. The fifth and last auxiliary right of the subject, that I shall at present mention, is that of having arms for their defense, suitable to their condition and degree, and such as are allowed by law.
The English right and responsibility to bear arms had never been unlimited. In the 17th century, Catholics were forbidden to bear arms. In reality, nobody cared unless a rumor went around that they were stockpiling arms. Similarly, the American colonies made arms mandatory for all but slaves and Indians, who were forbidden to have them, although slaves often used firearms, which were a common tool like a shovel on the frontier. Rich people in England wanted hunting reserved for them. For this reason, they got rules made specifying a minimum income that you needed in order to be allowed to have arms. There were restrictions on handguns and crossbows, which were compact enough to be concealed, and were often used by highwaymen. Guns a full yard long were OK.
The US 2nd amendment simply broadened the traditional rights of Englishmen and got rid of most of the exceptions. In historical context, the point of the clause about the militia is very clear. They wanted a militia instead of oppressive things such as select militias, mercenaries, or a standing army.
(This answer is based on notes I took on a book I read, but unfortunately I have lost the title of the book, so I don't have any specific references to cite here. However, I don't think any of this is particularly controversial, although its application to modern gun-rights politics may be.)
Just a supplement to @BenCrowells excellent answer, it was partly based on the English Bill of Rights 1689. The Roman Catholic James II had attempted to disarm Protestants, and set up a standing army - anathema to the English at the time. The right to bear arms in the Bill (actually limited to Protestants) was a reaction to a perceived (and probably actual) threat to Protestantism by James (who had all the diplomatic skill of the proverbial bull in a china shop!) The relevant text reads:-
Whereas the late King James the Second by the Assistance of diverse evill Councellors Judges and Ministers imployed by him did endeavour to subvert and extirpate the Protestant Religion and the Lawes and Liberties of this Kingdome (list of grievances including) ... by causing severall good Subjects being Protestants to be disarmed at the same time when Papists were both Armed and Imployed contrary to Law, (Recital regarding the change of monarch) ... thereupon the said Lords Spirituall and Temporall and Commons pursuant to their respective Letters and Elections being now assembled in a full and free Representative of this Nation takeing into their most serious Consideration the best meanes for attaining the Ends aforesaid Doe in the first place (as their Auncestors in like Case have usually done) for the Vindicating and Asserting their ancient Rights and Liberties, Declare (list of rights including) ... That the Subjects which are Protestants may have Arms for their Defence suitable to their Conditions and as allowed by Law.
The historical context shows that it intends that everyone be armed, both for the defense of the state and for their own personal use; that the "militia" is intended to consist of all capable adults; that broad membership and independence from a centralized army is the very thing that makes it "well-regulated"; that people were afraid of the federal government raising an army that out-gunned the general populace; that an armed populace was considered a bulwark for other individual rights, to the point of being used as a defense against the government if it should ever take a turn to the tyrannical; and that the right to be armed was widely considered a natural right on a par with free speech, a free press, etc. A few quotes from contemporary writings:
Congress shall never disarm any Citizen unless such as are or have been in Actual Rebellion
That the people have the right to bear arms for the defence of the State; and, as standing armies, in time of peace, are dangerous to liberty, they ought not to be kept up; and that the military should be kept under the strict subordination to, and governed by, the civil power.
— North Carolina Constitution of 1776 (Section 30 in the linked version, except the last sentence there wasn't present in 1776). Similar language is also found in the minutes of the constitutional ratifying conventions of Virginia and New York.
And that the said Constitution be never construed to authorize Congress to infringe the just liberty of the press, or the rights of conscience; or to prevent the people of the United States, who are peaceable citizens, from keeping their own arms
— Samuel Adams, in the Constitutional convention of Massachusetts.
There are other things so clearly out of the power of Congress, that the bare recital of them is sufficient. I mean "the rights of conscience, of religious liberty — the rights of bearing arms for defence, or for killing game — the liberty of fowling, hunting, and fishing..." These things seem to have been inserted among their objections, merely to induce the ignorant to believe that Congress would have a power over such objects, and to infer from their being refused a place in the Constitution, their intention to exercise that power to the oppression of the people.
— Alexander White, in a response to a minority (anti-federalist) opinion of the constitutional convention of Pennsylvania. The minority opinion said that the Constitution was insufficient because it didn't protect individual rights well enough (i.e. they wanted a Bill of Rights baked into the Constitution). White says that such protection is unnecessary because it's blindingly obvious that the federal government has no right, and no power, to curb such individual rights, even without their being listed specifically in the Constitution. Both sides of this argument were pro-right-to-bear-arms; they differed on whether a "Bill of Rights" was a good thing (that would protect individual freedom), or a bad thing (that would erode every freedom not specifically listed).
If the representatives of the people betray their constituents, there is then no recourse left but in the exertion of that original right of self-defense which is paramount to all positive forms of government
If circumstances should at any time oblige the government to form an army of any magnitude that army can never be formidable to the liberties of the people while there is a large body of citizens, little, if at all, inferior to them in discipline and the use of arms, who stand ready to defend their own rights and those of their fellow-citizens. This appears to me the only substitute that can be devised for a standing army, and the best possible security against it, if it should exist.
What, sir, is the use of a militia? It is to prevent the establishment of a standing army, the bane of liberty [...] Whenever governments mean to invade the rights and liberties of the people, they always attempt to destroy the militia, in order to raise an army upon their ruins.
America of the 1780s had to worry about a variety of enemies, both internal and external.
In some ways, the America of that time was much like today's "Switzerland" (which has universal male conscription and a militia army). Although (mostly) not mountainous, America was a Confederation of states that had won their freedom against common and powerful foreign enemies (mostly England, but also France and Spain in the French and Indian War). As such, it needed to worry about defense from those potential enemies.
As for "internal" enemies, Americans on the frontier had to worry about "Native Americans." And most Americans in the South worried about slave revolts.
A standing army was anathema to a people that had just fought for and won their independence. Today, America has a volunteer army. But in those days, just about everyone had, or could get access to a gun, and the ability to shoot. The natural consequence was the fact that America relied on mostly militia "armies" for most of its early years, basically until the Civil War. "All men are created equal" (except for slaves) extended to the military. Unlike the Swiss version, the American militia was a "volunteer" militia. No one had to join, but most were allowed to, if they wished.
What was the historical context of the 2nd amendment to the US Constitution?
Second Amendment Text
"A well regulated Militia, being necessary to the security of a free State, the right of the people to keep and bear Arms, shall not be infringed."
The controversy of the second amendment was about the "well regulated militia". The bill of rights was proposed by the anti federalists. The anti-federalists lead by Thomas Jefferson, had opposed the U.S. Constitution and the strong central government which it detailed. They feared the Constitution did not go far enough to protect individual liberties and they demanded a Bill of Rights to safeguard those liberties. The Federalists lead by George Washington generally believed the Bill of Rights was unnecessary. They used the opportunity of the ratification of the Bill of rights to insert things which they wanted, such as the second amendment. The second amendment is noteworthy because it contains a federalists core desire, professing the necessity of a well regulated militia. Something the anti federalists opposed, and something General Washington and his federalists had been lobbying Congress for. At the time the bill of rights was signed the United States had no standing army, nor navy, nor did they have a militia nor reliable access to the state's militias. The second amendment was a step along the path of establishing the US military, which was very controversial in the late 1700's.
Sept 3, 1783, Revolutionary War Ends, Congress disbands the Army and Navy
March 4, 1789, Constutiton is Ratified
April 30, 1789, George Washington first day as President
Dec 15, 1791, Bill of Rights is Ratified
Sept 29, 1793, Congress passes first bill to create a standing Army, or "well regulated militia".
Sept 30, 1793, George Washington's first day of second term in office
The "controversy" at the time of the founding fathers was the "regulated Militia" not the bear Arms which is the controversial today....The Second Amendment was a strategy to provoke a debate and drive the country in a particular direction by the Federalists. The first such debate had occurred two years earlier with the ratification of the US constitution, but this effort was a draw. The Constitution allowed Congress to fund a military, but not for longer than two years.
The 2nd amendment represented a second attempt to drive this point home only this time instead of the Constitution a document the federalists supported, they placed it in the bill of rights a document the antifederalists supported.
Richard Henry Lee.
Thomas Jefferson on Standing Army, 1789
There are instruments so dangerous to the rights of the nation, and which place them so totally at the mercy of their governors, that those governors, whether legislative or executive, should be restrained from keeping such instruments on foot, but in well-defined cases. Such an instrument is a standing army.
Congress had disbanded the United States revolutionary army and navy at the end of the Revolutionary war(September 3, 1783). When the United States Constitution was ratified March 4, 1789, it granted the congress the ability to fund a standing military, but Congress took no action on this right.
When President Washington took office April 30, 1789, the federal government had only a few dozen men under arms. They were used for guarding strategic locations like West Point. In case of war, the President was able to call upon State Militia's, 4 of which maintained their own standing armies, and raise an additional 700 men. The States, however had to give their consent. The States could deny the President access to their troops; thus state militia's were not reliable.
Congress did not change this law until the last day of George Washington's first term in office September 29, 1793. On this date 4 years after the Constitution was signed and two years after the bill of rights and after much debate, congress finally passed a bill to maintain the first peacetime army.
The battle which initiated the American Revolution, was sparked when it was discovered that British troops were about to be sent out to capture or destroy an arms cache which included cannons from the colonists.
About 700 British Army regulars in Boston, under Lieutenant Colonel Francis Smith, were given secret orders to capture and destroy rebel military supplies reportedly stored by the Massachusetts militia at Concord. Through effective intelligence gathering, Patriot colonials had received word weeks before the expedition that their supplies might be at risk and had moved most of them to other locations. Wikipedia: Battles of Lexington & Concord
The major reason was prevent the formation of a large, politically dangerous standing army. The experience of England, then Britain, as compared to the countries of the continent in the previous 200 years had demonstrated that if a monarch had a standing army, he could disregard the will of the people and make himself an absolute autocrat/dictator.
All the major kingdoms on the Continent, France, Spain, all the Germanies etc once had representative bodies like the English parliament. One by one they fell to the power of the king and his standing army. With a standing army, the key could just take without consent.
England dodged that bullet by being an island. An Island creates special defense problems, particularly back in the Age of Sail:
Ships were the fastest form of transport and a seaborne attack in one area could outrun the warning to another area.
Ships could switch their point of attack if repelled or the weather turned or for other reasons.
Ships could quickly mass at any given point on the coastline more man and fire power than any land force. If a serious attack occurred, the local resources that were on immediately on hand (like a fort) or with a couple of hours of travel would be all that could used to repel the attack.
All these factors played a large part in the successive waves of invasion prior to 1066. Although no one succeeded in landing a large force after the Normans, there were dozens of near successes that failed owing to weather, the actions of the British Navy or the enemy fear of that navy. Ireland was invaded several times by the French. Once the evaded the English Channel fleet, they had a smooth run.
Clearly, the only land force that could actively defend in that environment would to arm the general population so they could resist landings immediately. In the mid-1800s, they even created a militia of gunboats kept socked away near but usually not in or on likely landing areas.
America faced the same problem of having a huge area and boundary to defend in a time when it took weeks to travel from New York to Philadelphia. Any attack would have long failed or succeeded before Washington even knew the attack had occurred. Consequently, a coastal defense navy, a small standing army defending choke points and providing an officer core around which an larger army could be built in a real war, combined with a militia (which meant at least all enfranchised males) was the only conceivable defense strategy.
But perhaps more importantly, the Founders knew of the circumstances of English Civil War. It began with King Charles trying to like a sole autocrat who used imprisonment, torture and executions to enforce his breaking of law after law.
The war itself proved more than a little farcical, at least in the beginning, because the only Englishmen with landwar experience were those who had gone to fight on the continent. Most English cities still had medieval defense works which had not been updated to the new earthern star forts.
But with the political and social structure disrupted, a higher degree of meritocracy grew within the Parliament's side especially among the the religious non-conformist. With a few years, the New Model Army evolved into the world's most effective standing land army...
...and promptly took over the government. Just as many had history of England and the Continental experience warned, a standing army could crush the Constitution and Liberties of any land. If Oliver Cromwell had been a different sort of man, he could have murdered the English representative democracy and England would have continued to be dictatorship with parliament fading away to a mere ritualistic body just as it did in France. Fortunately for the rest of Anglosphere history, he spend most of political capital trying to find a way to restore the Parliament and restore representation.
But everyone in England had learned the lesson of the political dangers of standing army.
After Cromwell, with Parliament restored and the weasel Stewarts back on the throne, the first thing the Parliament did was to pack the new Model Army off to Ireland, Tangiers and various colonies. They did so to prevent the Stewarts from using them like Cromwell had. It clear that if Charles II would not have that James the II would.
This was the political history lesson that the Founders learned: A standing army will eventually lead to dictatorship. The lessons of the revolution itself seemed to prove it as the Revolutionary war pitted the rebels against professional soldiers from Britain and King George's German lands (the Hessins.) It took little imagination to project that lesson onto America itself with the President or a General using a large standing Army to overthrow democracy.
Battles like King's hill and the First Saratoga demonstrated that civilians equipped with civilian hunting rifles could harry and sometimes defeat trained soldiers armed with military muskets. The difference being virtually same between a modern deer rifle and a military assault rifle i.e. long-range, accurate but slow fire, and with poor close contact performance vs short-range, in-accurate but fast firing with good close contact performance i.e. bayonets.
The Founders had just come out two decade long struggle in which they saw a once trusted government suddenly begin to violate long established rights, then deploy professional soldiery against the citizens who could only begin to exist because they had large numbers of private weapons on hand.
Put plainly, without private weapons, there would have been no Revolution, not Constitution and no United States.
There was ever any serious debate in any of the States about whether citizens had a natural right to be armed. The major constitutional debate centered on what powers were to be granted to the Federal and State Governments to take control of the militia. Remember, this is time when you could walk into a store and by ship and field artillery with no paperwork. Merchant ships had to be armed and any private or locally managed fortifications needed cannons as well. Plus we had this vast frontier where people were always on their own.
To be frank, there is a lot of flat out lying from people who question original intent in the 2nd amendment and other parts of the Constitution. They try to talk about it like it some sacred text handed down out of the dawning mist of time from some unknown source and therefore requires massive interpretation to make some sense out of the whole thing.
That's a lie. The Constitution is just 240 years old, not 2400 years. More importantly, it is likely the most widely debated and voted on political document ever produced by any polity in the whole of history. Every single word or phrase was thrashed out, in public, for months or years before the people voted on it. In the case of the 2nd Amendment, every phrase in it had a concrete definition, widely understood by the ratifying public and there is no ambiguity at all in the 2nd Amendment. Those who say that the constitution requires some near mystical interpretation by modern priest class are lying.
Of course, that doesn't really matter to they post-modern Judiciary. Consider the legal term of art in the 8th Amendment:
Excessive bail shall not be required, nor excessive fines imposed, nor cruel and unusual punishment inflicted.
The phrase, "cruel and unusual" had a iron clad definition in English law for more than two centuries at the time. A "cruel" punishment was one disproportionate to the crime itself e.g. life imprisonment for a parking ticket, while the "unusual" meant that people who committed the same crime should receive the same punishment. The reason for the phrases use was to prevent the government from singling out individuals it disliked for disproportionate punishment that they inflicted on no one else.
In no way did the phrase invite philosophical debate about the nature of punishment itself. The phrase is there solely to protect specific individuals in specific cases for being treated differently.
But that's what the court did in Furman v. Georgia which halted executions in the US resulting in dozens of innocent deaths from the freeing of those once headed for execution (Kenneth McDuff in Texas being a good example.)
Once someone starts trying to debate whether the death penalty itself is at issue in the 8th Amendment, they're lying about amendment. And no there is no room for debate on the facts. You can spend literally years reading all the debates that lead up to the ratification of the U.S. Constitution. We know exactly with no ambiguity what the every jot and tittle meant when the people gave their "consent of the governed" when they ratified it.
People who argue otherwise are elitist who a tiny minority to be able to bend, warp and outright destroy any part of the Constitution they wish on their least whim with no restraints. The Constitution has a built in mechanism for its change and evolution, the elitist just can't use it because deep down, they have no respect for the people and are so impressed by their own brilliance they can't imagine they need anyone else's input. They don't want to spend the years it takes to get everyone to agree to a new amendment, they just want to change it as they wish, when they wish.
These are the people the Constitution was designed to stop. We have the 2nd Amendment precisely because the Founding generation foresaw exactly this circumstance. We've had 240 years to change it if majority thought we needed to and we have not done so.
Judicial fiat does not represent the "consent of the governed" and we have within a hair's breath of sliding into a Judicial Depotism in which the law is whatever the judges say it is, not whether what the people voted on.
Important to remember that laws are as written, not as discussed prior to signing. So, the points of view expressed during debate are only points of view, not a matter of law. Same with the history in England. It's true that the founders were, in fact, British citizens subject to British law until the moment that they rejected that status, but once they became "Founders" with the intention of creating something new, that vantage point must be considered to supersede the ideas of the past. They were either well informed or not well informed about the centuries of British history when they discussed, wrote and ratified the 2nd amendment, but they were certainly focused on getting something passed that addressed their immediate concerns.
The Federalist papers prove that the ability of the founders to predict the future regarding militias, and therefore the need for an armed populace, was imperfect. According to the rules as written at the time leading in the civil war, the South was within its rights to secede, and that did not go as it should have with a literal reading. Now, most states don't even have militias, so a literal reading of that amendment doesn't make sense. If people would step back from their locked and loaded convictions about what they believe is true or should be true, they might find interesting information from a close reading.
There are numerous interpretations, depending the outlook of the interpreter.
The term 'well regulated militia' is probably the most puzzling to us today. One one hand, this could indicate the old British habit of drawing it's bowmen from the civilian population, that armed citizens would form the militia.
During the revolutionary war, armed citizens played a role, including woodsmen with their very accurate (for it's day) long rifles, but not a large one. Most of the fighting was army on army, with the standard issue weapon being the Brown Bess, captured by the colonial armies from British stores. Irregular militias comprised of citizens with their own arms did exist, but weren't a deciding factor in the outcome of that conflict.
Or, given that a war of revolution against the British had just been fought, with the initial actions being undertaken against the British militias by armed citizens, the preamble may indicate that citizens can keep and bear arms to give them the ability to oppose a militia. In other words, it was the militias that might need regulation by armed citizens.
The context in which the 2nd amendment was written is not entirely clear.
The 2nd amendment was written 240 years ago and so needs to be interpreted from a historical perspective. In 1780 there was a need for a people to own guns for food and self-protection. The wording of the amendment specifically refers to militias and the need to have community based militia as a safety measure against an internal or external standing army. This was the key purpose and point of having the second amendment. So that it would be very difficult for democratic rights of the population to be subverted or overthrow by an autocratically controlled standing army. It was to provide a bulwark against erosion of democracy.
What America now has is a totally undemocratic private ownership of guns that is far and away out of context with the initial purpose of the 2nd Amendment. Now an individual has the ability to exert a mortal pressure over the rest of the population. The possession of highly sophisticated weapons far exceed those of a Kentucky light rifle that was in the original authors minds when writing the 2nd Amendment. The ownership by an individual of a complete arsenal of automatic weapons is a gross distortion of the 2nd Amendment.
A literal interpretation of the 2nd Amendment should dictate that weapons ownership is only available to members of local militias. Governments have the power to dictate that only certain types of weapons can belong in private individual possession. Further expansion of weapons technology and the private ownership of such weapons will only result in total anarchy and the destruction of the state.
I support the 2nd Amendment but not at the current and future cost it will demand.