North Korean leaders believed that the United States wouldn't intervene.
North Korean persons were assured when they moved to the Korean War because they evaluated the United States would not intervene.
The Korean War was started in the year 1950 and continues till 1953. This war was begun when North Korea's Political leader army tracked the 38th similarity and attacked non-Communist South Korea.
When the Korean War began, North Koreans were confident because they believed the US would not intervene.
Therefore, option D is correct.
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