Final answer:
The events of December 7, 1941, when Japan attacked Pearl Harbor, compelled the United States to abandon its neutral stance and enter World War II, creating a significant shift in the conflict.
Step-by-step explanation:
On December 7, 1941, a pivotal event altered the trajectory of World War II: Japan's surprise attack on the U.S. naval base at Pearl Harbor in Hawaii. This attack forced the United States to enter the conflict, thereby moving from a stance of neutrality to active involvement. President Franklin Roosevelt declared this date as "a date which will live in infamy" and, subsequently, on December 8, 1941, called upon Congress to declare war on Japan, which they did in a nearly unanimous decision. Just days later, on December 11, Germany and Italy declared war on the United States, which led America to declare war on them, too. The entry of the United States into World War II created a significant shift, as it transformed two separate wars into a connected global struggle and marked the end of American neutrality.