Since there had not been any offensives launched by either side against the other, why declare war just because Japan had attacked the USA?
There was an 88-minute long speech made by Hitler to the Reichstag on December 11th, 1941, which was four days after the Japanese attacks on Pearl Harbor, where he officially declared that Germany would join Japan in the war against the USA. In this speech, he mentioned a few of his personal reasons for this decision. That decision to declare war had been delivered to the Americans two hours before the speech by his foreign minister.
About two hours before Hitler began his address to the Reichstag, Germany formally declared war against the United States when Reich Foreign Minister Joachim von Ribbentrop delivered a diplomatic note to the American Charge d'Affaires in Berlin, Leland B. Morris.
I think this would be an interesting read regarding this question, and there's a translation by a Mr. Mark Weber on the Website of the Institute of Historical Review
Institute of Historical Review is considered by many to be a revisionist, Holocaust-denying institution with links to neo-Nazi organizations. Their translation of Hitler's speech might have been manipulated for propaganda purposes.
The US was already in a naval war with Germany (and not doing real well), was supplying all sorts of arms, supplies, and even warships to Britain, and was flagrantly violating the laws of war applicable to neutrals. Hitler was expecting war at some time in the near future, and chose to declare war first.
Hitler was also counting on the Japanese Navy to at least neutralize the US Navy, and considered the US to be a racially mixed and hence weak nation.
During World War II, American aid to the Allies fell under three categories:
- Lend-lease aid to Britain and Russia, of an amount roughly equal to the whole of the German war production,
- Fighting Japan, Germany's major ally, and
- the introduction of ground troops into western Europe.
American "Lend Lease" efforts had troubled Hitler and his admirals all through 1941. This was particularly true after American forces occupied Iceland, and the U.S. extended its zone of protection that far east, meaning that Britain would have to "cover" a relatively short part of the North Atlantic route on its own. Hitler could barely restrain his admirals from attacking U.S. ships.
Basically, Hitler could not win the war without the aid of Japan, because Germany was not strong enough to fight Britain and the Soviet Union alone (at least not after the latter received Lend Lease aid). Hitler could win if Japan could break the back of Britain (in India) or the Soviet Union (in Siberia). And it appeared to Hitler that Japan had, in fact, done this to the United States with its stunning (if incomplete) victory at Pearl Harbor. Journalist William L.Shirer reported in "the Rise and Fall of the Third Reich" that Hitler exulted, "we cannot lose this war, for we have an ally that has not been defeated in 350 years."
Hitler believed that the Japanese had started the process of destroying the U.S. navy, and that his submarines could finish the job. With the U.S. supposedly helpless, Germany and Japan could divide up the eastern hemisphere before finishing their dealings with the Americas.
On the other hand, if the United States fought Japan, that country would not be able to help Hitler finish off Britain and the Soviet Union. If the United States defeated Japan, and allied with China and/or India, Hitler could not win the war even if he conquered Britain and the Soviet Union. Hitler's "best chance" was to help Japan contain the United States in exchange for Japan's help in the eastern hemisphere.
Because Germany, Italy and Japan were allies, and fighting common enemies together is exactly what a military alliance is about.
By the way, the USA actually wanted to get involved in WWII, because they really didn't like how Germany was conquering most/all of Europe. Pearl Harbour and the Germany/Japan alliance acted as the classic casus belli.
In addition to the lend-lease reasons given by others, Japan had declared war on Britain at the same time as it declared war on the United States. Japan including the British Empire on the war declaration against the United States had to be reciprocated on the German side of the alliance with a declaration of war against the United States (since Britain and Germany had already been at war since 1939 at that point). With Japanese attacks on Malaya and Singapore, threatening Australia further south and India to the west, British ships and imperial troops (mostly from Australia) had to be withdrawn from the Mediterranean and North Africa and redeployed to the Pacific. It continued the pressure on Britain's links to its imperial possessions and dominions in the Indian and Pacific Ocean, which had been a major objective of the North Africa campaign (i.e. cut off the Suez Canal). With the Empire in the Pacific largely unprotected, Germany may have hoped Japan would be able to quickly neutralize the British Empire in the Asia Pacific region.
At the same time, in the Atlantic, Germany could now target American shipping to Britain far more openly. If the Japanese could win quick victories in the Pacific, and the US was slower to react in the Atlantic, the "tit for tat" war declarations (Japan on Britain, Germany on the USA) could have forced Britain out of the war through cutting it off from the Empire and the USA alike. With Britain out of the war, it would have been much harder for the USA to actually become involved in the European theater, and as for the Pacific - well that was always going to be Japan's problem anyway.
I think you have to consider Hitler's decision in conjunction with his antisemitism and the issues with which he was struggling in early December 1941.
Hitler did not believe that it was in Britain's interest to fight Germany, and that leading government figures, such as Churchill, were backed by "the Jews" of London and manipulating the country. Likewise he believed the USA to be in thrall to "the Jews" of New York.
Hitler also believed that "the Jews" were behind "Bolshevism" in the Soviet Union. Communism was the primal victory of "Jewry's" drive for state decomposition. Consider the purpose of the original Einsatzgruppen was to follow the Wehrmacht and kill "political commissars" which also meant "male Jews" and soon also women and children.
In this ideological space there was a direct push and pull between bombs dropping over Bremen, for example, and the Eastern Front. Or Lend-Lease aid to the Soviets, and so forth. The nexus of that "push and pull" was, for Hitler and all too many Germans, "the Jews."
In the time between October and December, Hitler and co. were debating whether to unleash the "Final Solution" ASAP or "in the spring, after the war." Meanwhile they were "researching" the ways and means of mass death in Auschwitz. A conference was scheduled in Wannsee on December 8 to discuss the coordination of the effort.
First came the Soviet counteroffensive in Moscow on December 5 and then Pearl Harbor on December 7, both of which came as a surprise to Hitler, and threw Hitler into a bit of a stun. It is not known whether it was a euphoric stun or depressive stun - different reputable historians have it going either way with equally convincing arguments. The historian I trust most on this matter, Christopher Browning, is on the euprhoricist side. It rings true with my own impressions that Nazis were optimistic about the war through 1942, got kicked in the stomach at the start of February 1943, and were glum by August 1943.
Hitler emerged from this stun talking for the first time of Weltkrieg (world war). He accepted that the war would last into 1942 and beyond. He declared war on the USA so that his U-boats could get to work, and gave his go-ahead to begin the Final Solution of the Jewish Question while the military aspect of the war was still underway. The Wannsee Conference was rescheduled for January 20, 1942, and work commenced on building the death camps.
In case you were wondering how it makes "military sense" for Germany to devote so many resources to the murder of millions of Jews, remember for the Nazis that was the entire point of the war: to "liberate" Germany from the "Jewish world conspiracy" and win the Lebensraum needed to ensure national survival. It was not a side issue - it all hangs together.
Hitler, not being over-informed in US history or culture, probably felt such warfare was inevitable, and, indeed felt that the multicultural USA would indeed be a weak opponent, and that the prime German war aim would be done by the time they actually came to blows.
Hitler thought that by declaring war on the USA, he would persuade Japan to declare war on the USSR in turn. This did not happen, and it is one more strategic mistakes of Hitler.
Hitler, while he had a vision and a goal, didn't plan things far out; rather, he wanted to take advantage of opportunities. He saw the attack on Pearl Harbor as one. Expecting that America would focus on the more direct aggressor, Japan, he declared war on America.
In the war to that point, Pres. Roosevelt wanted to help Britain, and did not want to see her defeated. There were various types of aid, but not much direct involvement. He could not go to war against Germany without good reason, which Hitler handed him. The rest, as they say, is history.
Hitler truly believed that if he went ahead and took on the United States now that that his ally (Japan) was at war with them that they would do the same with his enemy the Soviet Union. At that point he very badly needed to relieve the pressure on his soviet front lines. However the leaders of Japan were nowhere as rash & impulsive (and may I say stupid) as Hitler. They were not about to do something like declare war on two super powers in the same week. As it would have it the tactic might have worked.
To take out Poland, Hitler linked with the country on the other side of it and thus sandwich attacked his target. He declared war on USA because he thought Japan would then attack Russia, enabling him to repeat his previous successful tactic.
So I think what is most critical in understanding December 7th, 1941 is that Japan itself did NOT declare War on the United States but instead did knowingly launch a "sneak attack" on the US Navy and Army Air Forces on that day. There are those even at the time that the whole thing was a "contrivance" created by the Roosevelt Administration but the fact remains as FDR pointed out in his dramatic address that a "State of War now EXISTS" (emphasis mine) "between the United States and Japan."
This did give pause to the 3rd Reich as indeed they were under no obligation to support a "sneak attack" and indeed had many reasons to oppose an outright Declaration of War on the USA given such a circumstance.
That makes this a very valid question from an Historical point of view relating to any War but especially World War 2. I'm not sure there is any answer actually other than there were mutual declarations of War by the 3rd Reich and the USA against each other....which was great news for Russia and Great Britain....both of whom looked on the verge of defeat in the Winter of 1941.
There was nothing that came out during the Nuremberg Trials that I am aware of either on this question...even though the Nazi "Secretary of State" Von Ribbentrop was one of the captured and accused and was directly questioned by the English Jurist on this matter.
Hitler did anything to avoid war with America, up to a limit. Effectively, the USA was already at war with Germany. They extended their coastal zone of control to half way the Atlantic ocean. US warships interfered with U boat actions. Position of U boats were transmitted in clear text, so convoys could act on it. Sometimes that lead to open warfare.
When Japan struck at Pearl Harbor Hitler probably thought open war was preferable to armed US neutrality. Given the effectiveness of operation Drum beat, not really a bad decision.