What did the united states join the allies and help to win world war 2?

Answer 1

I think you wanted to ask why did the US join the allies etc,tough question because the USA at the start of WW2 was against the idea of involvement in another European war,both the government and the general public,however the German blitz on London gained Britain much public support in the US,also US ships were being sunk by UBoats in Germanys attempt to starve Britain inflaming public opinion,by now the US was by now acting beyond its neutral position and supplying Britain with war goods,so the stage was set,the Japanese attack on pearl harbour in dec 41 did the rest,with the US now at war with Japan,Hitler now declared war on the US,so now the US that had been torn between its policy of isolation,and sympathy for Britain and China(China had been invaded by Japan 2 yrs before the war in Europe) was now commited to the cause of ending German and Japanese aggression,finding common ground with Churchill US president Roosevelt now found himself also allied with Stalin the leader of the USSR that had been struggling for survival after Hitlers invasion of Russia, So it was Hitlers mistake to declare war on the USA at a time when Britain was still holding out,and the soviet armies were giving the Germans their first major setback at the gates of Moscow,Roosevelt had promised the US public he would keep them out of the war,but in December 41 first Japan then a few days later Germany gave him no choice,

Answer 2

We joined the war after Japan bombed Pearl Harbor.