President Obama’s critics have pounced on his use of the phrase “red line” to urge military intervention in Syria. They argue, in the words of Sens. John McCain and Lindsey Graham, that the “credibility of the United States is on the line.” Presumably they mean that Iran, North Korea and others are watching.

Recall how another American president dealt with a crisis of credibility. In 1983, just after the bombing of the U.S. Marine barracks in Lebanon, Ronald Reagan was certain that staying involved militarily was “central to our credibility on a global scale” and “vital to world peace.” Credibility concerns were paramount during the Cold War. Policymakers feared that the Soviet Union would interpret U.S. vacillation in Lebanon as a sign of weakness and an invitation for Soviet aggression. Secretary of State George Shultz warned that if the United States refused to stand fast, “I shudder to think what kind of a world of anarchy and danger our children will inherit.”

But a few months later Reagan withdrew, “redeploying” the Marines to ships off the Lebanese coast. Asked during a news conference what had happened to American credibility, the president responded, “We may have lost some with some people, but situations change.” In fact, within a few years, the Soviet Union began its slow-motion surrender, ending the Cold War on America’s terms.

It’s not clear how a deeper U.S. involvement in Lebanon’s bloody civil war, which lasted 15 years, would have strengthened American credibility. If keeping your word, intervening and staying the course help in deterring rogue nations, the United States’ decade-long interventions in Iraq and Afghanistan should have stopped the Iranian and North Korean nuclear programs. In fact, those interventions did not even deter our allies in the Pakistani government from aiding the Taliban.

Political scientists have studied the subject of credibility extensively. In “Calculating Credibility,” Daryl Press looks at the 1930s and the Cold War, periods when leaders felt the need to follow through on tough rhetoric for fear of losing face. Press concluded that “The evidence in this book suggests that the blood and wealth spent to maintain a country’s record for keeping commitments are wasted: when push comes to shove, credibility is assessed on the basis of the current interests at stake and the balance of power, not on the basis of past sacrifices. . . . Leaders understand that no two crises are sufficiently alike to be confident that past actions are a reliable guide to the future.” Another comprehensive study on “reputation,” by Jonathan Mercer of the University of Washington, reached similar conclusions.

Obama may have spoken too loosely about a “red line” in Syria. But the most damaging thing he could do now would be to take action simply to follow through. One does not correct for careless language through careless military action.

Syria is a humanitarian nightmare, which the United States should do more to address. Washington should help create and sustain more havens — in Jordan and elsewhere — for refugees and should coordinate with other countries to get aid in faster and more effectively to those in need. It is trying to bring the various rebel groups into a more coherent opposition movement, though that is a daunting challenge.

But we must understand that the Syrian conflict is fundamentally a civil war between a minority elite and the long-oppressed majority — similar to those in Lebanon and Iraq. People fight to the end because they know that losers in such wars get killed or “ethnically cleansed.” The only path to peace in such circumstances is through a political accord among the parties. Otherwise, intervention that helps the rebels win will end only the first phase of the war. The ruling Alawite minority would be toppled, but because they know they stand to be massacred, they would continue to fight ferociously as insurgents. The next phase of conflict could be even bloodier — with the United States in the middle. Remember that the first phase of the Iraq war — the overthrow of the minority regime — was tame compared with phase two, when the minority fought back as insurgents.

Secretary of State John Kerry is right to try to achieve such an accord and to enlist Russia. Such an agreement might now have to include an Alawite enclave in Syria because it is hard to see how the communities can peacefully live together. (The model here might be the Kurds in Iraq.) Obviously the odds are against a peace accord, and clearly the Russians have their own interests. But without some political agreement, military intervention will not end Syria’s humanitarian nightmare. It will only change its composition.

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Read more on this issue:

Anne-Marie Slaughter: What Obama should keep in mind as he weighs action in Syria

Eugene Robinson: Questioning intervention in Syria

Richard Cohen: Obama out of line