The simple answer is that they were attacked by a member of the Axis powers (Japan) and declared war on them. The illicited a declaration of war by Germany, Italy, Hungary, Romania, and Bulgaria. The longer answer involves years of relations and trade with the British, Hitler's seeming hatred of the US, after effects of World War 1, and other political issues dating back into the 19th century. Answer(2) Japan declared war on Russia and Germany betrayed Russia. I don't know about you, but I think that Russia no longer trusted Germany and wanted the allies to help them.
The greatest , most overwhelming reason, is Pearl Harbor. Because the US declared war on Japan , Germany and Italy ( Japan's allies) declared war on the US and things just went on. Of course there are many more reasons , that one is by far the greatest reason.
I do not know why won't someone tell me?
I didn't fight at all, which side did you fight on.
No, they were allies in WWII.
world war 1, and war war 2
Instead of obtaining allies when a war erupted; NATO allowed us to already have allies to fight a war, anytime it started.
No. The US fought against Hitler. We were allies of the UK.
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I didn't fight at all, which side did you fight on.
No, they were allies in WWII.
No. Actually they were part of the Allies in WWII.
The Allies
Leaders knew that if they declared war, their allies had to join the fight with them.
The Allies .
world war 1, and war war 2
To fight for their cause, as the Allies fought for theirs.
Germany, Italy, Austro-Hungarion Empire
In World War I, the Japanese joined the Allies for a time, and fought the German army in east Asia.
Yes, the US were part of the Allies during WWII.