The Acheson Defense Perimeter Speech was one of the most important factor contributing to the Korean War in 1950-1953. However, there are also other important factors such as Kim’s aggressive move to invade the South and US’s policy of containment. In January 1950, the US secretary of state, Dean Acheson, did not include South Korea in his list of countries that the USA would automatically defend against any communist aggression in his Defence Perimeter speech. Truman supported him in believing this mainland Asia was outside the USA’s Pacific defensive perimeter. It had been argued that since Korea was not mentioned as within the defence perimeter, it convinced the communists that the US was not ‘interested’ in defending South Korea. Hence, although Kim had urged an attack since early 1949, Stalin only agreed to the attack after the trade was made. This is a precipitating factor that led to the Korean War since the decision made by Dean Acheson seemed to encourage Kim to finalize an attack on South Korea, making him more aggressive on this move. The withdrawal of the last American forces from Korea, as well as North Korean Kim's conviction that the US would not intervene, convinced the North Koreans to attempt to unify the country by force. The Soviets, led by Stalin, and the Chinese, led by Mao, concurred with both Kim's judgement about the United States and his plans to unify the country by force. In June, he struck, without much hesitation. Without this Defence Perimeter Speech, Stalin would not have approved Kim’s decision to attack South Korea and the conflict might not have escalated into a war. Another reason for the Korean War was due to US’s policy of containment. In line with Truman’s policy of containment, the USA decided that communism should be contained on the periphery of Asia- China, South Korea. He rushed supplies to South Korea and took advantage of the absence of Russia in the Security Council to push UN to take action. Truman believed that Kim was acting as Stalin’s puppet in an attempt at world domination and that if communism was not stopped in Asia, there would be a domino effect. The communist victory in China, combined with the first Soviet nuclear tests in 1949, resulted in a new US policy of containment in Asia. This policy which became known as the defensive perimeter strategy included drawing up war plans to defend a belt of offshore Pacific islands in Asia against any further Soviet expansion. The USA also maintained or built airbases and garrisons on the islands to make effective military resistance. President Truman’s immediate reaction was to both Kim's judgment and conviction that the US would not intervene. The last reason for the Korean War is Kim’s aggressive move to invade the South. Historical documents seem to imply that Kim was the driving force behind the North’s decision to invade the South. Though Stalin and Mao were aware of Kim’s plan, they did nothing to discourage him. In fact, Soviet military experts helped draw up the plans and sent advisers as well as equipment to Kim. However, Stalin made it clear that if US got involved, the USSR would not intervene directly to help Kim. China had its own problems with her new communist rule, in addition to Taiwan, thus, Mao made no firm guarantees of military assistance. This is an important reason as Kim’s aggressive attack directly led to the war. It plays an important role as Kim’s aggression and self confidence prompted him to attack the South Korea without much hesitation. Kim was confident that with Rhee’s unpopularity and evidence of growing support for the communist party in the South, most Koreans would see him as a national hero if he reunited the country. He resented the USA’s post-western regional power and, mindful of US backing for Jiang Jieshi in China’s civil war, feared the same assistance might be given to South Korea. Kim being the risk-take that he is, decided that was best to invade the South, before the US would turn their attention from Japan.