When North Korea invaded South Korea on June 25, 1950, the United States in an apparent reversal of its policy decided in a single week to commit its combat forces to Korea. This quick decision, tersely known as the “Korea Decision,” begs a question. How did it happen? To answer the question this paper employs the cognitive process model of decision-making. Unlike some earlier analyses that deduce the decision directly from the belief systems of the key decision makers, the model focuses on how the pre-decision variables, including the “operational code,” interact with the decision-making process variables to lead the key decision-maker(s) to a particular decision. To look into the black box of President Truman’s mind, the paper relies primarily on the so far largely untapped private letters of Truman. For day-to-day developments, I relied on the Glenn Paige’s classic, The Korean Decision. In the final analysis, the most important factor was the decision-makers' perception of the situation which, in turn, was conditioned by their “operational code” and the decision-making situation as represented by he “reinforcements/ feedbacks.” Throughout the whole stages of decisions, the perceptional framework changed little, but was reinforced. Their simplistic perceptional framework limited the range of alternatives, which were always put in two extreme forms, forcing upon the decision-makers an “either-or” choice. With their perceptional framework also producing a single value at each stage, the selectable alternative was almost always predetermined. Besides, the political setting consistently reinforced the decision in one direction. The support provided by the public and the Congress whenever a firm action was chosen prodded the decision-makers toward an even firmer posture at the next stage of decision. The closedness of decision-making process as indexed by the number of decision-makers also increased the propensity to biased perception.