It's interesting to note that the 'red line' failure Galen mentions in his "Mark Halperin was right" column is hardly the first such failure. A similar situation rather arguably led to the NORKS invading South Korea on 25 June 1950. Dean Acheson, then Secretary of State, gave a speech in January 1950, in which he defined our 'defensive perimeter.' He left Korea out of it. So the NORKs, the Soviets, and the Chinese Communists figured we didn't think South Korea was worth a fight:
Secretary of State Dean G. Acheson's speech at the National Press Club on 12 January 1950 was among the most important and controversial US policy statements in the early history of the Cold War in East Asia. In it, he defined the American "defensive perimeter" in the Pacific as a line running through Japan, the Ryukyus, and the Philippines. This denied a guarantee of US military protection to the Republic of Korea (ROK) and the Republic of China (ROC) on Taiwan. Less than six months later, North Korea launched a military offensive across the 38th parallel that nearly succeeded in imposing Communist rule over the entire peninsula. Critics immediately pointed to Acheson's National Press Club speech as giving Pyongyang the "green light" to pursue forcible reunification, based on the premise that the United States had ruled out military intervention to defend South Korea. More than fifty years after the start of the Korean War, countless South Koreans still hold Acheson responsible for igniting this fratricidal conflict. The United States, they bitterly maintain, committed an act of betrayal toward Korea ranking with President Theodore Roosevelt's approval of the Taft-Katsura Agreement in 1905 and President Harry S. Truman's agreement to divide the peninsula forty years later at the end of World War II.