Although Japan's attack on Pearl Harbor came as a shock, tensions between Japan and the United States had been building for decades.

The United States was especially displeased by Japan's growing aggressive behavior toward China. The Japanese government saw territorial expansion and monopolization of the neighboring country's import market as the only way out of its economic and population decline. The Nanking Massacre and other crimes were committed after Japan declared war on China in 1937.

The United States government retaliated to this hostility with a series of trade and economic embargoes. They reasoned that cutting off Japan's supply of money and goods, particularly necessities like oil, would force the country to rein in its expansionist tendencies.

Instead, the sanctions hardened Japanese resolve to maintain their position. Months of talks between Tokyo and Washington, D.C. resulted in little progress. Conflict appeared to be imminent.

Hawaii's Pearl Harbor is about 4,000 miles from Japan and 2,000 miles from the continental United States, in the middle of the Pacific Ocean. The idea that the Japanese would strike the faraway Hawaiian Islands to begin a war seemed laughable.

In addition, American spies were certain that any attack by the Japanese would be launched against one of the European possessions in the South Pacific, such as the Dutch East Indies, Singapore, or Indochina.

Due to the lack of preparation by American military leaders, Pearl Harbor's naval facilities were vulnerable to attack. Around Ford Island in the harbor, the majority of the Pacific Fleet was docked, and nearby airfields were crammed with hundreds of planes.

Pearl Harbor was a tempting and simple target for the Japanese.

Nearly 20 American ships and over 300 planes were damaged or destroyed as a result of the Japanese attack on Pearl Harbor. Airfields and dry docks were also wiped out. Most significantly, over 2,000 lives were lost.

Unfortunately, the Japanese Pacific Fleet was not disabled. By the 1940s, aircraft carriers had surpassed battleships as the most significant type of naval vessel, yet on December 7 that year, none of the Pacific Fleet's carriers were stationed at their home base. Others were ferrying planes to the troops on Midway and Wake Islands, while still others had returned to the mainland.

Furthermore, the base's oil storage depots, repair shops, shipyards, and submarine docks were all still operational after the attack on Pearl Harbor. The U.S. Navy was able to recover from the attack fast because of this.

Over 2,403 Americans, including sailors, soldiers, and civilians, lost their lives in the Pearl Harbor attack. Additionally, 1,178 people were injured. A total of 129 Japanese soldiers were slain. When Pearl Harbor was attacked, the USS Arizona took the brunt of the casualties. The sinking battleship is now a memorial to the lives lost by the United States in the attack.

The day after the devastating attack on Pearl Harbor, President Franklin D. Roosevelt addressed a joint session of the United States Congress.

As President Franklin D. Roosevelt put it, "Yesterday, December 7, 1941—a date which will live in infamy—the United States of America was suddenly and deliberately attacked by naval and air forces of the Empire of Japan."

He continued, "The American people in their righteous might will win through to absolute victory," regardless of how long it took to overcome this deliberate invasion. When I say, "We will defend ourselves to the uttermost, and we will make very certain that this sort of treachery shall never threaten us again,” I feel I am interpreting the will of the Congress and the people.

Despite years of debate and discussion, the Pearl Harbor attack finally united the American people behind the idea of going to war.

Japanese leaders had hoped to provoke the U.S. into agreeing to end economic sanctions against their country, but instead they drove its foe into a worldwide battle that led to Japan's first occupation in almost a century.

The declaration of war against Japan by President Roosevelt was ratified by Congress on December 8. Japan's allies in Europe, Germany and Italy, declared war on the United States three days later.

Once again, Congress responded by declaring war on Europe. U.S. involvement in WWII began more than two years after the war had begun.