But success raised a question: What were the allies fighting for? The 38th parallel had been the dividing line between North and South. Soon that status quo would be restored. What should the U.S. do then?

The U.S. could pause and propose a cease-fire. It could pursue the NKPA in order to capture and disarm it, then withdraw. It could occupy North Korea and proclaim a unified Republic in the entire peninsula. Or it could drive on to the Yalu.

A few voices in Washington warned that the drive to the Chinese border was risky. The Chinese, through diplomatic sources, said that an advance on the Yalu would bring them into the war. U.S. military leaders, under the influence of MacArthur, belittled China’s determination and capabilities. The troops rolled on toward the Yalu—with no clear announcement of why they were going there or what they would do.

A few weeks later, Chinese “volunteers” poured over the border, catching U.N. forces by surprise. MacArthur had publicly promised GIs that they would be “home for Christmas.” But they were pushed back down the peninsula to a shaky truce line near the parallel—where they held, fighting a bitter shadow war while generals tried to negotiate a settlement. Eight thousand five hundred Americans had died in Korea before the Chinese intervention. More than three times that number died in the next two years.

In retrospect, it’s not clear which choice would have been the correct one. Pausing at the border might simply have allowed the NKPA to regroup and attack again. Occupation might have been costly, and installing the Rhee government in the North—already criticized for its autocratic ways—might have destroyed the unity and legitimacy of the U.N. effort. And proceeding directly north posed a threat of war with China.

But what should concern Americans today is that a handful of powerful men in the U.S. military and the White House made the decision. The National Security Council warred with the Joint Chiefs of Staff about limits on the military once it entered the North; MacArthur seemed to have his own, more expansive ideas. U.S. allies expressed concern through diplomatic channels; but at no time was there a public debate.

The U.S. military action was taken under United Nations Security Council Resolution 83, which called on member states to “furnish such assistance to the Republic of Korea as may be necessary to repel the armed attack and to restore international peace and security in the area.” Whatever that meant, it was the only public statement of American war aims.

Congress had not been asked neither when U.S. troops were sent into the war in June 1950, nor when U.N. forces crossed the parallel in October. The White House ordered troops and materiel into the fight without even consulting congressional leaders. A few old Washington hands—such as Ambassador Averell Harriman—urged President Harry Truman to seek formal authorization from Congress. Some Republican leaders, like Senators Robert A. Taft of Ohio and Kenneth Wherry of Wisconsin, asked him to follow constitutional procedures; but others, like Senators Scott Lucas of Illinois and Tom Connally of Texas, opposed such a move. So did Secretary of State Dean Acheson. The fear was that an open debate would allow the far-right, McCarthy wing of the Republican Party to air their charges of incompetence and treason, sending a message of internal division at a time when unity seemed to be needed. Besides, the people were united—everyone believed that the aggression should be stopped.