Hitler and his rationale behind the declaration of war came down to the fact that the US would eventually join the war in Europe. With this in mind and Russia looking like it was about to collapse, Germany needed an extra push to knock the Russians out, and this is where Japan enters the picture.

Even though it has been meme’d to death, Germany and Hitler were not angry for Japan bombing Pearl Harbor. In fact Hitler was quite impressed and shocked Japan would strike such a decisive blow against the US. Hitler now saw an opportunity for Japan to join the war and possibly knock out the Russians. While Pearl Harbor was being attacked, the Germans were outside of Moscow. Taking Moscow would have, in the German general’s minds end the war. And while the opening stages of Barbarossa were extremely successful, the Red Army now over it’s initial shock put up better defenses and coordinated counter attacks. And by December 7th, the Germans were stuck on the outskirts of Moscow, what was worse was the Russian counterattack. German forces were pushed back from Moscow by a large Soviet attack, mainly from reformed units and Far East troops hastily brought to the front. Hitler now hoped the Japanese could launch a two front war against the US and USSR.

Hitler wasn’t foolish enough to disregard the US and it’s industrial might. He knew they would be drawn into the conflict in Europe soon enough. But before they could get there, he hoped to secure Europe before the US could arrive in force. Now with Japan attacking the US, this proved to be an opportunity for them. While the Germans and Japanese had a defensive pact amongst themselves, it didn’t guarantee a call of arms or aid if one of them attacked. But now Germany could use the rallying cry of a common enemy to grow closer to Japan. Declaring war on the US hopefully would bring the Japanese and Germans closer. Hitler now thought Japan could hold off the US and then attack in Siberia and pinning down the forces there. While Hitler and the Wehrmacht would secure Russia and it’s resources.

But there were a few problems:

1. Hitler overestimated Japan’s military might

2. The Japanese were not obligated to attack Russia or coordinated attacks

3. Japan had a neutrality pact with the USSR and had no desire to attack them

4. Japan had it’s own problems and had to deal with them first

Japan’s military situation in the Pacific was in a precarious position. As they were in a war with China, had forces tied up guarding the Manchuria/Russian border, plus planning an offensive on the South Pacific. Not to mention a shortage of most war materials and the US cutting off all oil shipments, Japan was doomed unless it got more oil. And the Dutch East Indies were a prime oil center in the region. Japan had no reserves or enough resources to plan their invasion of the Southern Pacific and an attack in the North while fighting in China.

Japan also got humiliated in a border conflict with Russia in 1938 in Lake Khasan and Khalkin Gol. Japan and the Soviet Union would later sign a neutrality pact. The Japanese High Command agreed they were outmatched by the Russians; and while they were a threat, they would avoid conflict with them unless the country was in complete chaos. Even at the height of Operation Typhoon, when the Germans were only 30KM from the Kremlin, the Russian defenses did not collapse. Russia after taking that beating in Barbarossa was not in chaos. The Japanese would adopt a wait and see stance and giving vague promises to the Germans they would attack Russia. This never came about as Japan successfully took British Malaya and the Dutch East Indies and had all the resources they needed. Even if they attacked the Russians, they would still need those resources that they were short on.

Hitler declared war on the USA to try and bring together the Japanese and Germans and unite against a common enemy. But this was never going to happen as the Japanese had their own problems they needed to care of first.