1941年7月28日，日本無視美國的警告，進駐中南半島南部。美國於7月31日凍結了日本在美國的資金，隔日並全面禁止輸出任何石油到日本。日本與美國隨即進行談判，然而一直無法達成共識。1941年12月8日，日本偷襲珍珠港正式向美國宣戰，展開了為期四年的太平洋戰爭。 直至1945年二戰結束，陸海軍一直都是與內閣有同等地位的機關，因此在決定許多重大政策時必須要採納陸海軍之意見。1941年10月東條英機出任內閣總理大臣兼任陸軍大臣後，陸海軍對政策的影響及發言權更為顯著。 戰爭影響人民的生命財產非常重大，展開太平洋戰爭應是慎思熟慮的結果，本論文研究戰前陸海軍的決策過程，以期了解其開戰的原由。 結果發現，太平洋戰爭的啟動，並非全然在周密計劃下執行；而是陸海軍在國內的立場、組織的存在意義、物資、預算、軍備、動員計畫等所謂的組織利益相互衝突下，所漸漸形成的。 / On July, 26,1941, ignoring the warning from the U.S., the advance of Japanese troops into Indo-China of French colony forced the U.S. to freeze the Japanese Capitals in USA few days later. On August 1, 1941, the U.S. imposed an embargo on the export of oil to Japan, who immediately started to negotiate with U.S. They, however, couldn’t reach any kind of consensuses. December 8,1941, Japan declared war against the U.S., and started the Pacific War that lasted for 4 years. Before the end of WWII in 1945, the Army and the Navy ranked as high as the prime minister. Therefore, for decision-making on many policies, the opinions of the Army and Navy must be taken into considerations. October 10, 1941, Tojo Hideki was chosen as the prime minister and the general of the imperial Japanese Army, which intensified the influence of armed forces on the decision-making processes. Wars have a very strong impact on people’s lives and properties. The decision of starting the Pacific War should have been a decision made after deliberation. Through analyzing the decision-making processes between the Army and the Navy, this thesis aims to understand the reason of waging war. The result revealed that the declaration of Pacific War was not well planned. It was the conflicts of organizational benefits between Amy and Navy, such as domestic political positions, the meaning of existence of the organization, raw materials, budgets, armaments, mobilization plan, etc. that gradually leaded to the Pacific War.