By providing weapons, money and food to the UK during the war America was already essentially part of the war. They weren’t contributing their blood, sweat and tears but pretty much everything else.
After Pearl Harbor they were full blown participants, but they didn’t need to be... They could’ve concentrated on taking on Japan and and ignored the European theatre and Hitler, but Hitler made the mistake of aligning himself with Japan in 1936. That sealed his fate and made America’s declaration of war against Germany a fait accompli.
The Anti-Comintern Pact (1936) and Tripartite Pact (1940) were key Axis alliances with different focuses: the first was a 1936 German-Japanese agreement specifically targeting the Soviet Union/communism, while the 1940 Tripartite Pact was a broader military pact among Germany, Italy, and Japan designed to deter the United States.
And the Hitler-Stalin Non-Aggression Pact pretty much killed the original Anti-Comintern Pact, as eventually Japan made their own deal with Stalin.