The attack on Pearl Harbor on December 7, 1941, was important because it sparked the United States' entrance into World War II. The day after the Japanese attacked Honolulu's Pearl Harbor, the United States declared war on Japan.
When the events at Pearl Harbor took place, World War II had already been going on for two years. Three days after President Franklin D. Roosevelt asked Congress to declare war on Japan, Japan's allies, Germany and Italy, declared war on the United States. Following this, Congress reciprocated, declaring war on both Germany and Italy. In the end, Japan's decision to attack Pearl Harbor left the United States with no choice but to enter the international conflict.
In the aftermath of the 2-hour attack on Pearl Harbor, 21 ships in the U.S. Pacific fleet had been sunk or severely damaged. United States air crafts also took a hit, as 188 were destroyed and 159 were damaged, most were hit before they could even leave the ground. In total, 2,403 people were killed, the majority of which were soldiers and sailors.
The surprise Pearl Harbor attack was also responsible for uniting the nation, which was split about whether or not to even enter the conflict that was World War II.