Short Answer 
On December 7, 1941, Japan launched a surprise attack on Pearl Harbor, leading to significant destruction and loss of life, prompting President Roosevelt to declare war and marking America’s entry into World War II. The attack united the American public, transitioning the nation from isolationism to a powerful military presence that significantly influenced the war and altered its global role.
The Surprise Attack
On December 7, 1941, Japan executed a surprise attack on the U.S. military base located at Pearl Harbor, Hawaii. This assault led to widespread destruction of the naval fleet and resulted in significant loss of life. The attack was executed with meticulous planning and aimed to incapacitate the U.S. Pacific Fleet, enabling Japan to expand its territorial ambitions in the Pacific.
Declaration of War
In the aftermath of the attack, President Franklin D. Roosevelt delivered a historic speech to Congress, describing the day as “a date which will live in infamy.” His speech galvanized public sentiment, leading Congress to swiftly declare war on Japan the following day. This declaration marked the end of America’s isolationist policy and signaled the nation’s formal entry into World War II.
Mobilization and Impact
Following the attack, the attack united the American public and exponentially increased war mobilization. The previously isolationist United States transitioned into a powerful military force, eventually engaging in battles on both the Pacific and European fronts. This involvement not only contributed to the downfall of the Axis powers but also reshaped America’s role in the global community, marking a significant turning point in its history.