Not trying to be Devils Advocate but the war started for the USA with Pearl Harbor or even earlier if you count the Lend Lease program. The war had been going on for some time when the USA joined. The Japanese had gotten off to an early start with Manchuria in 1931. People just didn’t recognize the war for what it was then much like what is going on today.
I was thinking more in terms of declared hostilities. IMO, the war really started the day that the British and the French forced the ridiculous Treaty of Versailles on Germany. When that document was signed, Round 2 was guaranteed.