A series of events led to the attack on Pearl Harbor. War between Japan and the United States had been a possibility that each nation's military forces planned for since the 1920s. Real tension did not begin until Japan’s1931 invasion of Manchuria. Over the next decade, Japan expanded slowly into China, leading to an all-out war between the two in 1937. In 1940 Japan invaded French Indochina in an effort to block all supplies coming into China, including war supplies purchased from the U.S. This move prompted the United States to place an embargo (block on trade) on all oil exports to Japan, leading the Imperial Japanese Navy to estimate that it had less than two years of oil remaining. Japanese officials needed to seize oil resources in the Dutch East Indies. The embargo also showed Japan that America was not willing to tolerate Japanese expansion into China and Southeast Asia.
Japan's 1937 attack on China was condemned (complete disapproval) by the U.S. and several members of the League of Nations including Britain, France, Australia, and the Netherlands. These states had economic and territorial interests, or formal colonies, in East and Southeast Asia; they were increasingly alarmed at Japan's new military power and its willingness to use it, which threatened their control in Asia.
Dutch East Indies Philippines The Philippine islands, at that time an American territory, were also a Japanese target. In July of 1941, American and Filipino forces in the Philippines were combined under American General Douglas MacArthur. With the addition of U.S. B-17 bombers stationed in the Philippines, America hoped to dissuade (convince not to) Japan from attacking the Dutch East Indies. The Japanese military concluded that an invasion of the Philippines would provoke a war with America, because American forces in the Philippines were mobilizing. The mobilization of U.S. troops in the Philippines worried Japanese leaders. Rather than seize and fortify the islands, and wait for the inevitable US counterattack, Japan's military leaders instead decided on the pre-emptive Pearl Harbor attack, which they assumed would eliminate the American forces needed for the liberation and re-conquest of the Philippine islands.
In 1940, Japan signed the Tripartite Pact with Nazi Germany and Fascist Italy, officially forming the Axis Powers. Japan took advantage of Hitler's war in Europe to advance its ambitions in the Far East. The Tripartite Pact guaranteed each nation assistance if attacked by any country then considered neutral. This message pointed directly to the United States, and gave Japan more power on the political stage. The Tripartite Pact now threatened the U.S. on both shores of its continental span—Hitler and Mussolini on the Atlantic Ocean, and Japan on the Pacific. The Roosevelt administration believed the American way of life would be at risk if Europe and the Far East were to fall under military dictatorship. Roosevelt committed to help the British and the Chinese; he loaned money and materiel to both countries and pledged that U.S aid would be enough to ensure their survival during war. Doing so would slowly move the United States from a neutral country to one preparing for war.
With the British, French, and Dutch preoccupied with the European theatre of war, the main threat to Japan's planned invasion of Southeast Asia was the U.S. Pacific Fleet, consisting of three aircraft carriers, nine battleships, and scores of other ships. The Pacific Fleet had been based in San Diego, California but in 1940, as tensions continued to escalate with Japan, the United States decided to move the fleet to Pearl Harbor in the territory of Hawaii, which was several thousand miles closer to the hot-spots in Asia. This move was seen by Japan as an American step towards war.
The decision to move the fleet to Pearl Harbor was criticized by some American military leaders who claimed that Pearl Harbor was too shallow, that it was too far from the U.S. mainland and vulnerable to attack, that supplies (such as diesel oil for the ships) would have to be imported, and that its single channel could be a bottleneck for ships trying to get out to sea. Despite these objections, the Navy, in the spring of 1940, moved the U.S. Pacific Fleet to Pearl Harbor, a place many Americans had never even heard of.
In 1940, Japan moved into northern Indochina. This invasion, added to the Tripartite Pact, war with China, increasing militarization, and Japan's withdrawal from the League of Nations led the U.S. to embargo scrap metal shipments to Japan. In 1941, Japan moved into southern Indochina. The U.S. responded by freezing, on 26 July 1941, Japan's assets in the U.S. and, on 1 August 1941, embargoing all oil and gasoline exports to Japan. Oil was Japan's most crucial imported resource; more than 80 percent of Japan's oil imports at the time came from the United States. To secure oil supplies, and other resources, Japanese planners had long been looking south, especially the Dutch East Indies. The Japanese Navy was certain any attempt to seize this region would bring the U.S. into the war.
After the embargoes and the freezing of the assets, the Ambassador of Japan in Washington and the Secretary of State Cordell Hull held multiple meetings in order to discuss a solution to the Japan-American problems. No solution could be agreed upon because of three key reasons: Japan's alliance to Hitler's Germany and Mussolini's Italy through the Tripartite Pact; Japan wanted economic control and responsibility for Southeast Asia; and Japan refused to leave mainland China. Feeling the economic squeeze from the U.S embargoes, Japan had a sense of urgency, either it had to agree to Washington's demands and restore normal trade, or use force to gain access to the resources available throughout the Pacific. Japan started to believe that war with the U.S. was unavoidable.
The Pearl Harbor attack planning arose out of the Japanese expectation the U.S. would be inevitably drawn into the war after a Japanese attack in Southeast Asia. The intent of a preemptive (first attack) strike on Pearl Harbor was to neutralize American naval power in the Pacific, thus removing it from influencing operations against American, British, and Dutch colonies to the south. A surprise attack posed a twofold difficulty compared to long standing expectations. First, the US Pacific Fleet was a formidable force, and would not be easy to defeat or to surprise. Second, for aerial attack, Pearl Harbor's shallow waters made using conventional air-dropped torpedoes ineffective. On the other hand, Hawaii's isolation meant a successful surprise attack could not be blocked or quickly countered by forces from the continental U.S.
In November of 1941, an enormous naval force left Japan and headed for the islands of Hawaii. Japan felt that after the aggressive moves made by the U.S., war was unavoidable and they had to make the first move. American forces in Hawaii were not expecting the attack that was coming.