History of South America

What event made Americans realize the US must take a role in foreign affairs?

The attack on Pearl Harbor.

On December 7, 1941, the Imperial Japanese Navy launched a surprise attack on the United States naval base at Pearl Harbor in Hawaii. The attack resulted in the deaths of over 2,400 Americans and the destruction of much of the U.S. Pacific Fleet. The attack on Pearl Harbor shocked and angered Americans, and led directly to the United States' entry into World War II.