Hitler didn't have to declare war on the USA. He was just too overconfident.
From Wiki
[QUOTE][/Japanese declaration of war on the United States propelled, although it did not require, a similar declaration of war from all the other signatories of the Tripartite Pact.QUOTE]
The mistake the Japanese made was to attack the United States. They should have punched north from Manchuria, cut the Soviet Union in half enabling Germany to mop up the western side with the Russian Far East Army stuck in the east.
Then they would have got their oil supplies.