Navy's main concern was planning for any attacks from Japan in the Pacific. Ever since the open door policy, the relationship between the U.S. and Japan had been tense as Japan continued expanding its territory in the Pacific. Japan invaded Manchuria in 1931 to further expand Japan's territory. As a result of the increase expansion in China, Japanese aircraft launched an attack on the American gunboat USS Panay. Japan had numerous reasons for their recent behavior. The Japanese home islands did not contain many natural resources. Japan had no source of petroleum and had to depend on oil imports, which only came from the United States. As of result, Japan expanded into oil-rich nations such as Singapore, Thailand, Philippines, and Hong Kong. To hide these actions, Japan hide their plans under diplomatic negotiations. However, because the U.S. controlled the Philippines which lay directly north of the Dutch East Indies, Japan felt threatened that the U.S. could end its whole economy. This, however, only served to intensify the already tense relations between the United States and Japan. .
When France fell in 1940, Japan was able to occupy the remaining French colonies, this in addition to Japan's increasing presence in China forced the U.S. to increase measures to counter Japan's recent actions. As a result, the United States established a series of economic approvals designed to force Japan to reconsider their current and future actions. America's first step was to stop the sale of aviation fuel to Japan and also ban the sale of iron and steel. In response to the U.S. actions, Japan made an agreement with Germany and Italy known as The Tripartite Pact. However, Japan had already set in motion the war with the U.S. in the pacific. .
In September of 1941, the Japanese Army and Naval leaders threatened the U.S. that they would be hostile towards them if they did not change their demands. In the same month, General Tojo Hideki was named the new prime minister for Japan and also Japanese naval planners submitted an outline of a carrier base air surprise attack on the U.
