After Pearl Harbor, the United States went to war against both Japan and Nazi Germany. But there was a possibility that we might not have gone to war against Germany. If not, how might history have been changed?
Everybody knows the United States declared war against Japan after the Pearl Harbor attack on December 7, 1941. Most people know about President Roosevelt's speech about the "day that will live in infamy" as he asked Congress to declare war against Japan. Some people know that on December 11, Adolf Hitler declared war against the United States. But if Hitler had not declared war, would the United States have declared war against Germany? As far as I can tell, no one knows. Roosevelt had not asked Congress to declare war against Germany. If Hitler had not acted, we might have gone to war only against Japan.
Why did Hitler declare war? The Tripartite Pact, which said that Germany had to support Japan if Japan was attacked, did not necessarily obligate Germany to declare war if Japan initiated the attack. Of course Hitler was quite willing to ignore treaties anyway if they were inconvenient. So it looks like Hitler's declaration of war was a huge mistake. Germany was already in a desperate struggle with the Soviet Union. The last thing they needed was to get in a war with the world's largest industrial country.
Would the United States have declared war against Germany if Hitler had not acted? It is not clear that they would have. For instance, Churchill's memoirs said how relieved he was to have the United States in the war, declaring in typical melodramatic fashion how "We should not be wiped out. Our history would not come to an end. We might not even have to die as individuals." But he never even mentions that the US did not declare war until after Hitler did. The only thing he mentions is in a letter to the King on Dec. 8 is that he expected Germany to declare war on the US because of the Tripartite Pact. Churchill does not discuss if the the United States would have declared war if Hitler had not. Other sources I have seen talked more about Hitler's decision making process than what the Roosevelt administration would have done. But the sources I have seen are mostly popular treatments of history. What do the professional historians say? There must have been some discussion behind the scenes. Did Roosevelt try to find out what Congressional leaders were thinking on the subject? Were there diplomatic exchanges on the subject?
What if the United States had not gone to war with Germany in December 1941? History would have changed. Maybe the US would declared war later. They were already doing some protection of Atlantic shipping against the U-Boats. But when and how? Would the Soviet Union dominated much more of Europe without an invasion of Italy or France? Without a western front, could the Nazis have held out against the Soviets much longer, or even stalemated them? Could the Germans even have captured parts of the Middle East, and gotten access to oil resources? The US war against Japan might have been shorter with all of the US resources deployed in the Pacific.
This is a major historical turning point that may have depended on the whim of Hitler.