During the American Independence War the British army made use of mercenaries hired from various German state (About 30,000 German soldiers fought for the British during the American Revolutionary War, making up a quarter of all the soldiers the British sent to America).
Why? Were the Hessians better?
The British army simply didn't have enough soldiers available when the war started. Per the Wikipedia page, their total military strength was around 45,000 men, and Lord North and General Howe didn't think this was nearly enough to succeed. Toward this end, the parliament authorized the raising of 55,000 soldiers and 45,000 sailors in October of 17751.
The problem was, where do you find the man-power to more than double the size of your armed forces quickly? Recruiting in England at the time was extremely difficult and the British military was volunteer at that point. AmericanRevolution.org notes:
Throughout the war the government experienced great difficulty in obtaining sufficient men for the ranks. Again and again it was found impossible to complete the augmentation voted by parliament. The correspondence of the adjutant general, Edward Harvey, is burdened with complaints about the state of the recruiting. "Sad work everywhere in recruiting," he writes in December, 1775. "In these damned times we must exert zeal." The competition for recruits among the various regiments was intense. Some of them, not satisfied with such able-bodied men as they could secure by hook or crook, enlisted invalids and out-pensioners. Not a little ill-feeling was aroused among the militia officers by attempts to enlist their levies as well. Prior to 1775 Roman Catholics as a rule had been excluded from the ranks; but now those in Connaught and Munster were gladly welcomed. Recruiting parties were even sent into the American colonies. As is well known the paucity of men led not merely to the hiring of the Hessians, but to the recruiting of many Germans into British regiments. In 1775 bootless attempts were made to procure 20,000 mercenaries from Russia and the use of a Scottish brigade in the pay of Holland. These facts strikingly illustrate the appalling scarcity of available fighting men.
The decision to hire auxiliaries was not only driven by that necessity, it was also perfectly normal for the British military at the time:
All British wars in that century had been fought by contracting with continental princes. The officers in America expected this to happen. Gage recommended hiring foreigners. Captain William Glanville Evelyn of the 4th (King's Own) wrote to his cousin that the assistance of foreign troops would be highly politic and of these Russians were 'the most eligible, not only as being good soldiers, but by their not having any connections in this country, and from not understanding the language, they are less likely to be seduced by the artifice and intrigue of these holy hypocrits'.2
Note in both of the above quotations that the German mercenaries were not really the preference of the British as much as available to the British. Frederick II of Hesse-Kassel was not only more than willing to rent out soldiers to whoever could pay for them, but was also George III's nephew. Whether that eased the negotiations is hard to say, but at a price-tag of over £3,000,000 I would have to think it was more a business than family affair. This wasn't without controversy in either Hesse-Kassel or England:
Nor did this measurement escape severe animadversion in the British Parliament. It was warmly censured by many members of the opposition, especially by Mr. Adair and Mr. Dunning, who maintained that, in engaging the services of foreign mercenaries without the previous consent of parliament, ministers had violated the provision of the Bill of Rights, and by this infringement of the Constitution they had set a precedent which might be made available by some future arbitrary monarch to the destruction of the liberties of the country.3
1 Blake, Rev J. L. - A History of the American Revolution, p 177
2 Atwood, Rodney - The Hessians, p 23-4
3 Shepherd, William - A History of the American Revolution, p 87
As a broad generality, Britain is a naval power, not an infantry power. Britain is protect by "wooden walls". At the time of the US war for independence, Britain had just completed the Seven Years War and was trying to demobilize the officers from that war; Britain couldn't afford to pay half pay to their retired officers let along staff up a new military that could fight in America.
Strategically speaking Britain couldn't achieve its military objectives with a Navy. Britain needed to exert control over the colonies, extract wealth from the colonies, and develop a land policy that incorporated the settlement of demobilized Naval officers.
Exert control - critical strategic objective, but of little relevance to your question. It is worth observing that Parliament had only been sovereign since around 1700, and had only been managing an empire since about 1750.
Extract wealth from the colonies - The colonists were far better at smuggling than the British navy was at suppressing smuggling. And ultimately the colonists could ignore any interdiction and just live off the land. There were substantial numbers of the colonists who were content with the opportunity to own, develop and farm land - an opportunity that was categorically impossible in England. The only way to extract wealth was to exert strong executive control, and that requires infantry/cavalry/artillery.
Develop a land policy that accommodated not only the need to resettle demobilized Naval officers, but that respected the rights of the Native Americans. Britain had treaties with the Native Americans, and one of the causes of revolution was the colonial's refusal to abide by those treaties. Unless Britain developed an effective monopoly on the use of force in the colonies, there was no way that Britain could enforce their treaty obligations.
Hessians solved all those problems - they were far more skilled than a comparable British force would be (Britain would have to recruit and train to field a force that size) and they were far cheaper than staffing up an infantry (that would have to be demobilized at some point),
In 1776, Britain had a population of 9 million (and numerous commitments all over the world). "America" had a population of 3 million. This compares to 25 million in France, and an even larger number in "Germany" (taking into account all the German states). With "only" a three to one numerical advantage, diluted by the vast expanse of the Atlantic Ocean, England would have a hard time defeating "America" by herself.
On the other hand, Britain was richer than France or Germany (on a per capita basis). Hence, they had a "comparative advantage" in money, and a comparative disadvantage in manpower, and it made sense for them to pay money to hire other countries' men to fight for them. If it were not for the "extra" men we got from France, the extra men that the Germans provided probably would have made the difference in England's favor.
It may be noted that although popular US history often describes Great Britain as the mightiest power in the world at the time, Britain was rather puny in the size and power of its army.
The Chinese Empire and some other Asian powers had armies in the hundreds of thousands, and at least four European powers, France, Russia, Austria, and Prussia, also had armies in the hundreds of thousands and far better trained and equipped than Asian ones. Thus the major military powers of Europe had armies at least four or five times larger than Britain's.
Minor powers like the larger German states often had armies in the tens of thousands, and thus roughly in the same league as the British army.
Landgrave ("count of an entire land") Frederick of Hesse-Kassel sent the largest contingent of "Hessian" allies, but there were also troops from other states.
They included Hesse-Hanau, ruled by a son of Frederick of Hesse-Kassel, the Margravites ("border counties") of Brandenburg-Ansbach and Brandenburg-Bayreuth (ruled by a cousin of the King of Prussia), the Principality of Anhalt-Zerbst (ruled by the brother of Empress Catherine the Great of Russsia), The Principality of Waldeck, the Duchy of Brunswick-Wolfenbuttel (ruled by a cousin of King George III), and the electorate of Brunswick-Luneburg (Hanover) where negotiations were made easier by the Elector being King George III himself.
What the British really needed was to ally with one of the major military powers, such as France, Russia, Austria, or Prussia, to get a really large reinforcing army, but that did not happen.
The French were soon allied with the Americans, the Austrians were technically allies of the French and might possibly also have supported the Americans in slightly different circumstances, Austria and Prussia were distracted in 1777 by fighting the brief "Potato War", and the League of Armed Neutrality in 1780-1783, protesting the British searches of neutral shipping, eventually included Russia, Sweden/Finland, Denmark/Norway, Prussia, Austria, the Ottoman Empire, and both Sicilies, almost every major European state that wasn't already allied to or at war with Britain.
As a child in 1950's Britain, my school taught that the American Revolution was a revolution of Englishmen in America to preserve their historic liberties and rights against the attempt by the German (Hanoverian) King Georges I-III to impose a European-style monarchy on Britain. The story was that many British soldiers and even officers refused to go to America to fight fellow Englishmen trying to preserve the historic rights of Englishmen in Britain and North America. Hence the King's need for loyal "German" troops.
