It’s a powerful, reasoned position, amply supported by the history of America’s involvement in Vietnam. But that history also shows that a president’s attitude and analytical assessment, no matter how gloomily realistic, are not necessarily an antidote to ill-advised military action. Foreign intervention has a logic all to itself.

Today we think of Lyndon Johnson as a man unwaveringly committed to prevailing in Vietnam. But at least at first, he shared Mr. Obama’s pessimism. He and his advisers knew they faced an immense challenge in attempting to suppress the insurgency in South Vietnam. “A man can fight if he can see daylight down the road somewhere,” he said privately in early March 1965. “But there ain’t no daylight in Vietnam.”

Johnson also knew that the Democratic leadership in the Senate shared his misgivings, and that key allied governments counseled against escalation and in favor of a political solution.