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How did Dean Acheson’s 1950 speech affect debates about the Korean War’s origins?

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Korean War, June–August 1950 — Korean War Map showing North Korean advances in the Korean War in June–August 1950.

In his January 12, 1950, speech at the National Press Club, U.S. Secretary of State Dean Acheson outlined an Asian defense perimeter that included Japan, Okinawa, and the Philippines but notably excluded Korea and Taiwan.1

For years, conventional wisdom held that this omission "invited" the North Korean invasion, suggesting that the United States had signaled a lack of interest in the peninsula. However, the declassification of historical documents has led to a significant reconsideration of the war's origins. Evidence suggests that the State Department actually viewed South Korea as vital to the defense of Japan.1

Rather than ignoring the region, Acheson’s exclusion of Korea from the defense line likely indicated that the United States did not intend to station its own forces there and sought to avoid providing Syngman Rhee with encouragement to invade the north. Consequently, historians now argue that Acheson was attempting to prevent a conflict, while also ensuring that if hostilities did break out, the responsibility would clearly rest with the Communists.1