Final answer:
Before the United States entered WWII, factors like the rise of Adolf Hitler, the aggressive expansionism of fascist regimes, and the recovery from the Great Depression contributed to its involvement. President Roosevelt's support to the Allies and tensions with Japan also played a role, culminating in the catalytic attack on Pearl Harbor in 1941 that led to the US officially joining the war.
Step-by-step explanation:
Before WWII, various factors contributed to the US getting involved. During the interwar period, despite a strong inclination towards isolationism, factors such as the rise of fascist regimes and the aggressive expansionism of Germany and Italy led to international tensions.
Adolf Hitler's rise to power and the subsequent annexation of territories like Austria and the Sudetenland pressured the US to reconsider its isolated stance.
Moreover, the US economy, recovering from the Great Depression, began to grow through increased industrial production and exportation of military equipment, and consumer goods to the Allies.
President Franklin D. Roosevelt sought to aid the British against a potential German invasion and address escalating conflicts with Japan over power dynamics in the Pacific.
Ultimately, the sudden and devastating attack on Pearl Harbor by Japan on December 7, 1941, served as the immediate cause of US entry into World War II, although hostilities had been brewing for some time due to international affairs.