Pearl Harbor was attacked by Japan on December 7, 1941. Before Pearl Harbor, Japan and the US were not on good terms with each other. Japan had been a very isolated country for some time and when the 20th century began, they started to expand. They had two successful wars against China and during the Great Depression Japan sought to solve their economic problems by forcing their way into China. In July of 1937, another Sino-Japanese war began, and went on into a grueling six weeks of mass killings and rapes. (Nanjing Massacre)
Due to this the US passed economic sanctions against Japan. Japan then signed the Tripartite Pact with two other nations, Germany and Italy, fascist regimes. The US hoped their embargoes on Japan would encourage them to quit their expansion, but it did the exact opposite. Japan wanted to defend their status as a major world power. Japan's attack aimed to destroy the Pacific Fleet and destroy morale in the US Navy so America wouldn't be able to fight back.
Im sorry it was kind of long, if you need me to elaborate on one specific part or anything like that, please let me know, I am happy to help.
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