I just want to note that this might be the 1,600th question on this subreddit regarding Adolf Hitler according to /u/Demeche, so congratulations?

Anyway, of course we have plenty of documentation regarding Hitler's reaction to Pearl Harbor, the most important of which is the Declaration of War against the USA. Not the most "uh oh" type of reaction for sure when you go up to someone after they just got punched by your buddy, and then proceed to also punch them.

In fact, this was one of Hitler's rash and emotional diplomatic decisions, of which there we numerous in 1941, particularly his decision to expand Operation Marita, initially designed to support Italy in Greece by attacking Greece from Bulgaria, to also attack Yugoslavia after the coup d'etat by the Yugoslav air force officers. The expansion of Marita was one of the German generals' main alibis for blaming the war on Hitler, but the decision to attack the United States was less pronounced in these post-war calculations of how to blame everything that went wrong on Adolf Hitler. Still, it finds mention in several German generals' writings.

If, aside from Hitler's stubbornness, foreign policy played a decisive role in the decision making of these days [military decisions in the war against the Soviet Union, early December 1941] is outside of my knowledge. I'd like to assume so however, because on 8 December, Japan entered the war, which was followed on 11 December by the German declaration of war against the United States of America.

The soldier of these days wondered why Hitler declared war against the USA, but not inversely Japan against the Soviet Union. As a result of this [Japanese] inactivity, the Russian Far East forces remained free for usage against the Germans.

• Guderian, Heinz. Erinnerungen eines Soldaten.

I will once more return to the previous point with the declaration of war - I don't wish to criticize the historical accuracy of the greatly dedicated and learned 14-year-old history experts over on the likes of HistoryMemes (they are especially well-read on the histories of women, Islam and communism, among others), but these ones you reference don't seem to have gone through the appropriate amount of peer reviews to be classified as truly rare and dank. Hitler was under no "uh oh" obligations after Pearl Harbor, because a Japanese attack on the United States did not mean a war with Germany for America.

In fact, the Americans were quite distraught - they had wanted to help Britain and the Soviet Union in Europe, and the American public continued to be opposed to intervention in the war. If the US now turned around and used the Japanese attack as a justification to declare war on Germany, the public backlash could have been severe, as it might have confirmed the perception in parts of the American public about Roosevelt's irrational desire to draw America into war with Germany, especially now that the Americans really should draw together all their forces in the Pacific and smash the Japanese in a concentrated campaign without any European distractions.

And so, when Ribbentrop's declaration of war was read to US chargé d'affaires Leland B. Morris in Germany, the Americans were the ones that were relieved or even happy, for their deliberations regarding the war (which had been made in secret for most of 1941 as the US Armed Forces prepared to enter the war on the side of Canada and the UK) could now be put into effect in the way they desired, in a 'Germany first' strategy.

When Pearl Harbor happened, we were desperate. [...] We were all in agony. The mood of the American people was obvious – they were determined that the Japanese had to be punished. We could have been forced to concentrate all our efforts on the Pacific, unable from then on to give more than purely peripheral help to Britain. It was truly astounding when Hitler declared war on us three days later. I cannot tell you our feelings of triumph. It was a totally irrational thing for him to do, and I think it saved Europe.

• Galbraith, John Kenneth. 1995 interview with Sereny, Gitta.

But apart from these geostrategic considerations among Hitler's military leaders and Roosevelt's advisors, what about the man himself? Well, Hitler and Ribbentrop had been in long negotiations with Japan's foreign secretary Yosuku Matsuoka in spring of 1941. The German strategy had always been to reduce the number of Germany's enemies and to take them on one by one, one front at a time. The negotiations with Matsuoka had preceded Barbarossa, it is worth pointing out. Well, the strategy for the Germans to delay the US entry into the conflict was to have Japan decisively crush the British and Dutch colonial empires in East Asia and to thus divert America's main attention to the Pacific. If, furthermore, Britain could be defeated on the whole - by the German actions in North Africa and Iraq, combined with Japanese attacks on Singapore, Malaya and Australia -, the Axis hoped that the Americans could be kept out of the war for good until the present conflict was concluded. By then, they had already made the decision that the United States would fall under attack soon after the victory over Britain and the Soviets.

However, as part of his negotiations with Matsuoka, Hitler made one of his bellicose speeches that turned into a verbal pledge of assistance: If Japan were to get into a conflict with the United States, Germany on her part would take the necessary steps at once. Germany would promptly take part in case of a conflict between Japan and America. And, to answer the question that was evident in Guderian's excerpt further up, they did not only make this bellicose promise to Japan, but they failed to get Japan to make the same promise in regards to Germany and the USSR. In fact, Matsuoka would just a few days later sign a neutrality agreement with the Soviets in Moscow. Once more, the foreign policy department around Ribbentrop, who was perhaps the least qualified and least intelligent foreign secretary of all the major powers of World War II, had totally failed. Germany's subsequent attempts to induce Japan to attack the Vladivostok area during Germany's invasion of the Soviet Union would prove fruitless: The Japanese observed their neutrality agreement with the USSR until 1945, when Stalin himself would become the one to break it and have the Red Army invade Manchuria, not only to defeat Japan, but also to install communist regimes in China and Korea.

And Hitler's grand strategy went downhill in December 1941, when the Battle of Moscow concluded on the 6th and became the first major operational failure for the German armies during the campaign, but also when Japan attacked the USA on the 7th. The Japanese were playing their own game, and Hitler's hopes that they would keep the Americans out of the war had backfired spectacularly. Japan, threatened by American economic embargoes, saw itself forced to lead a swift pacific campaign in the hopes of crushing America's Pacific empire as well as making multiple advances on British and Dutch colonies in South East Asia. With a decisive victory over the US Navy, a seizure of all of America's Pacific possessions and eventually a destruction of the Panama Canal, the Japanese hoped to permanently disable the American threat to their imperialistic Pacific ambitions.

And then, Pearl Harbor came. Matsuoka, the ambassador Hitler had made his promise to, had left the Japanese government - but Ribbentrop had made a new promise on 29 November 1941, one that the Americans intercepted. The attack on Pearl Harbor hit the Germans just as much of a surprise as it did the Americans. Germany's own secret war that they hadn't told Japan about - the one against the Soviet Union - was ongoing and had just turned bad for the first time since the campaign's beginning, and Japan had decided to ignore Germany's appeal for assistance in this case as well. Germany seemed to be in no moral or tactical need to actually declare war on the US: The undeclared conflict between US Navy vessels and German submarines would of course continue in the North Atlantic, as would American lend lease shippings to the UK and later, through Iran, the USSR, but there was no reason to enable the US to bring hundreds of thousands of American troops to the European warfront against Germany. The war would firmly unite the US legislators behind their President in wartime solidarity and allow Roosevelt to initiate any military conscription, economic consolidation, government spending or strategic action he and his generals might deem necessary, meaning that the full might of the American industrial giant was now on the table against any of America's enemies.

So why did Hitler add himself to that list of enemies? Well, he firstly thought that the reverse was inevitable. Spurred on by America's undeclared war in the Atlantic, he was convinced that the Americans would surely now make the war into a declared one as well. There was great concern among the Americans that there was a potential that the democratic institutions in Washington would block Roosevelt from entering the war against Germany, but Hitler did not recognize this potential. In reality, it is quite likely that Congress would have granted Roosevelt and his foreign secretary Hull the declaration of war they wanted on the Axis, but they had intercepted a message by Ribbentrop assuring German support to Japan on 29 November 1941, and so the American leaders decided to wait for Germany and Italy to make the first move, hoping it would strengthen the case for war among the US public. They had learned from the Nazis, who used the Briitsh and French declarations of war on 3 September 1939 constantly in their own propaganda. And they were right: America found itself "on the defensive" once again, on 11 December 1941, when Germany and Italy declared war on the United States of America.