Because The Japanese attacked them at Pearl Harbor on December 7th, 1941. :)
allies
The US joined the war on the side of the Allies on April 6, 1917.
After the US joined WWI, the allies had the forces they need to triumph. The axis powers fell and the war ended.
Saudi Arabia joined the Allies on 1 March 1945.
The Germans
Germany tried keeping the US out of the war in England by bribing Mexico with states like Arizona, New Mexico, and Texas, which were former Mexican Territories. Germany knew that when the US joined the allies that they would in sense "clean house." In 1917, when the US joined the Allies, the allies forced Germany and Austria-Hungary to give up.
He joined forces with Germany in WWII against the US and allies
No. The US fought against Hitler. We were allies of the UK.
The US gave the Allies a fresh army. Right after the US joined Russia dropped out. This gave the Allies a dominant chance to win. Without the US, Russia could have stayed and finished off the Allies without the US.
The united states joined the war because Germany sank the famous ship the Luistania (killing hundeds of Americans) and cargo ships with submarines. The u.s. joined the allies because they were fighting the Germans.
US wouldn't have joined the Allies in WW2 and the Allies would have probably lost.
The good ole USA