In order to answer the question, why did Pearl Harbor happen, it is important to understand the international dynamics that led up to one of the greatest military attacks of all time.
Near the turn of the 20th century, Japan was isolated and had spent much of its history literally and figuratively as an island. It had fought in two wars, one against China and the other against Russia, and it began to posture and expand its empire.
During the 1930s, Japan began to force its way into China and the League of Nations (LON), to which Japan belonged, issued strong condemnation of the occupation. Japan further isolated itself internationally by withdrawing from the LON.
However, this still does not answer the question, why did Pearl Harbor happen? In 1937, Japanese forces carried out several weeks of mass killings known as the Nanjing Massacre. Appalled by the atrocities, the United States leveled economic sanctions against Japan since China had friendly relations with the United States. U.S. support flowed to some Chinese factions/forces.
As international alliances grew against Japan, the nation entered the Tripartite Pact with Germany and Italy in 1940. Leaders in Japan and the United States unsuccessfully negotiated for a long time prior to the attack on Pearl Harbor. The relationship between Japan and the United States was tense and it had been contentious for at least a decade. Sometimes the U.S. role in Japanese provocation is overlooked when people ask why did Pearl Harbor happen?
The embargoes that were instituted by the United States did not work. Rather than lead Japan to halt its expansionism, the sanctions convinced Japan to dig in and stand up to what was considered western meddling in Asian affairs. In Japan, waging war against the United States was a foregone conclusion because the Japanese believed that they were defending their right to expand and grow as a world power. In addition, the United States did some posturing of its own. Why did Pearl Harbor happen? Mostly because moves made by the United States to dissuade Japan from further empire-building backfired. Rather than discourage, the actions provoked Japan.
In the United States, the government began conducting military exercises and ramping up war production in preparation for conflict, should it happen. The draft was also instituted in September 1940 and President Franklin D. Roosevelt ordered the U.S. Navy to move the Pacific Fleet from San Diego to Pearl Harbor. Hawaii was then a U.S. territory. Why did Pearl Harbor happen? It’s clear that Japan felt threatened.
Japanese Adm. Isoroku Yamamoto planned the Pearl Harbor attack aiming to surprise the American Pacific Fleet and destroy it so it would be unable to counterattack and defend expanding Japanese forces in the Pacific. Japan never intended to invade Hawaii. It simply wanted to cripple the American fleet so it could attack European colonies in Asia.
Initially, the Pearl Harbor attacks on Dec. 7, 1941, looked like a total success. Japan struck eight U.S. battleships, but they only sank four. The others were damaged, but reparable. The Japanese also destroyed more than 300 aircraft and killed 2,403 Americans; 2,008 were sailors; 218 were soldiers; 109 were Marines; and 68 were civilians.
However, the Japanese attacks on Pearl Harbor failed to destroy the Pacific Fleet and the Japanese missed key supply areas and logistical facilities. Most importantly, no U.S. aircraft carriers were at Pearl Harbor the day of the attack. The carriers USS Enterprise, USS Saratoga and USS Lexington were out on maneuvers and were not spotted by the Japanese, which was a stroke of luck for the United States. This would prove to be a pivotal failure for Japan and in 1942, U.S. carriers helped win the Battle of Midway.
In 1946, the Army tried to answer the question, why did Pearl Harbor happen? According to the Army's official history, “Guarding the United States and Its Outposts,” the Congressional Pearl Harbor Joint Committee concluded: “There was a complete failure in Hawaii of effective Army-Navy liaison during the critical period and no integration of Army and Navy facilities and efforts for defense. Neither of the responsible commanders really knew what the other was doing with respect to essential military activities.”
The failures were serious. In the Army’s report, why did Pearl Harbor happen had a few answers, but most notably these were important points of failure. For example, in the pre-dawn hours of the attack on December 7, a submarine periscope was spotted near Pearl Harbor, where there shouldn't have been any submarines. At 6:37 a.m., the destroyer USS Ward dropped depth charges, destroying the submarine. The incident was then reported to the Navy chain of command, but not to the other tenant service at Pearl Harbor, the U.S. Army.
Meanwhile, at the Opana Radar Site on the north shore of Oahu, radar operators Army Pvt. Joseph L. Lockard and Army Pvt. George Elliott detected an unusually large formation of aircraft approaching the island from the north at 7:02 a.m.
At the time, radar was experimental technology, and operators only monitored it from 3:00 a.m. to 7:00 a.m. Usually, the radar was shut off by 7:00 a.m., but the truck that took Lockard and Elliott to breakfast was late, so the radar was still on at 7:02 a.m.
The privates had never seen so many blips, so they called Army 1st Lt. Kermit A. Tyler, an Air Corps pilot who was an observer that morning at Fort Shafter's Radar Information Center in Oahu. He told the privates to ignore the blips, assuming the formation was a flight of B-17 bombers he knew were enroute from California that morning. The privates had failed to tell him how large the group of blips was and Tyler was also untrained in the new position. Why did Pearl Harbor happen? For many, the answer is the compounded mistakes of several people and leaders. But it was beyond just the innocent mistake of one man.
If the Army and Navy had been in communication, they might have recognized the signs of the coming attack. The sighting of a large aircraft formation coming in from the north and the sighting of a submarine at the mouth of Pearl Harbor could have been assessed. The Navy and Army could have done the math, shared intelligence, alerted forces and set up defenses. While Navy antiaircraft guns began firing soon after the attack started, Army guns did not come to life immediately because the ammo for them was locked in another location.
The first of two waves of some 360 Japanese fighters, bombers and torpedo planes began the attack at 7:48 a.m., having launched from six aircraft carriers north of Oahu.
Prior to the attacks, Army and Navy signals intelligence personnel were working hard to break the Japanese code. They were intercepting communications and decrypting what they could, but the communications they intercepted gave no clear warning of the impending attack.
The Japanese surrendered September 2, 1945. Why did Pearl Harbor happen? A lack of coordination and complacency that did not match the assertive diplomatic efforts of the U.S. government. Why did Pearl Harbor happen? The U.S. military was siloed, and many say that the Pearl Harbor attacks paved the way for what would be the birth of joint warfighting.