There were several events that convinced many Americans that the United States needed to prepare to enter World War II:
The German invasion of Poland in 1939: This event marked the beginning of the war in Europe, and many Americans saw it as a direct threat to the security and stability of the continent.
The fall of France in 1940: The rapid defeat of France by Germany was a shock to many Americans, and it raised concerns about the possibility of German aggression spreading across Europe and beyond.
The bombing of Pearl Harbor in 1941: This surprise attack by Japan on the U.S. naval base at Pearl Harbor, Hawaii, brought the United States into the war and galvanized public opinion in support of the war effort.
The rise of fascist regimes: The rise of fascist regimes in Europe, such as Nazi Germany and Fascist Italy, threatened democratic values and human rights, and many Americans saw it as a moral imperative to defend these values.
The lend-lease program: The U.S. government's decision to provide military aid to Great Britain and other Allied powers through the lend-lease program signaled a growing commitment to support the war effort, even before direct U.S. involvement.