The “day that will live in infamy” ended with the deaths of over 2,400 sailors, Marines and soldiers, along with the heavy damage and destruction of eight battleships. The surprise attack, conducted by hundreds of Japanese aircraft flying off of four heavy aircraft carriers, catapulted the United States into a world war it had been seeking to avoid.

But the attack, which left the U.S. population in a state of shock at the time, was one that was a long time coming. Japanese relations with the United States, which had enjoyed decades of peaceful cooperation, had been deteriorating for over a decade, and in the end, an U.S. oil embargo triggered the war.

Since Japan had invaded and occupied Manchuria in 1931 and left the League of Nations in 1933, it had pursued an increasingly aggressive foreign policy. Its imperial ambitions were directed at forming a “Greater East Asia Co-prosperity Sphere” — a euphemism for an empire modeled on the great European powers.

Japan gradually encroached on Chinese territory, and the incident on the Marco Polo Bridge in 1937 — where confusion over a missing Japanese soldier led to Japanese forces attacking the bridge — sparked an all-out war with China. The Chinese were riven with internal divisions, with a civil war between the Chinese Nationalists and the Chinese Communist Party leaving them too divided to successfully fight off the Japanese.

The Japanese occupation of China was savage, with indiscriminate reprisals carried out against Chinese civilians in revenge for partisan attacks. An estimated 20 million Chinese were killed during the course of the war.

The infamous Rape of Nanjing in December 1937, where hundreds of thousands of civilians were massacred and raped after Japan occupied the Chinese capital, sparked an outcry in the West. Britain, France, and the U.S. all sent aid to China, such as military supplies and the Flying Tigers, a U.S. volunteer fighter unit, and economic sanctions began to take their toll.

The Japanese turned to the Axis, signing the Tripartite Pact with Germany and Italy in September 1940. Its division from the Allied powers was complete.

But it was the Japanese invasion of French Indochina in 1941 that finally set the stage for Pearl Harbor. Though ostensibly “allowed” by German-occupied France to take control of the colony, it was too much for the United States. President Franklin Roosevelt instituted an oil embargo and froze all Japanese assets in the U.S. in order to pressure Japan to withdraw from its conquests. Britain followed suit as well.

This was a disaster for the Japanese economy, as it lost three quarters of its overseas trade and nearly 90% of its oil imports. To the Japanese, this left them with one option: knock the United States out of the Pacific with one massive blow and secure the oil and other resources it needed by occupying South East Asia.

After the devastation at Pearl Harbor, the Japanese enjoyed great success, invading and occupying Hong Kong, Singapore, Burma, the Philippines, Malaya, and New Guinea. But despite such great initial conquests, the Japanese had started a war they could not win.

The United States was the greatest industrial power on earth, the proverbial “sleeping giant.” With its full resources mobilized and larger population, the United States could simply out produce the Japanese in every kind of war material. In 1943 alone, the United States built over 85,000 aircraft, while the Japanese built only 16,000. In the numbers game, the Japanese were doomed.

Admiral Isoroku Yamamoto, the mastermind of the Pearl Harbor attack, said that in a naval war with the United States, “If I am told to fight regardless of the consequences, I shall run wild for the first six months or a year, but I have utterly no confidence for the second or third year.”

Considering the decisive Battle of Midway happened exactly six months after Pearl Harbor, where the Japanese lost four irreplaceable carriers and the tide of the war turned, Yamamoto was more prophetic than he might have wished.