After the end of World War I, Americans were not at all willing to engage in another international conflict. The war resulted in seemingly countless casualties and widespread despair, so many believed that it was a huge, costly mistake. With Americans keen to dodge any involvement in international affairs, the close of WWI marked the beginning of America's largely isolationist period. The nation signed a series of arms restriction treaties to limit the size of militaries and passed neutrality legislation in Congress to prevent arms makers from profiting by selling arms to countries at war.
In 1939, after France and the UK declared war on Germany, President Franklin D. Roosevelt indicated that America's isolationist stance might be ending. He stressed that even though the US was technically neutral, Americans should not close their consciences for the sake of neutrality. Though the US was neutral, it appeared the nation favored the Allied side.
Ultimately, the US entered WWII partially because they were forced to. In 1940, Germany prevailed over France, leaving Britain, a key American ally, to ward off Hitler's advances on their own. On December 7, 1941, Japan attacked Pearl Harbor. The very next day, the US declared war on Japan. A few days later, on December 11, the US declared war on Germany.