But in Syria, I fear prudence has become fatalism, and our caution has been the father of missed opportunities, diminished credibility and enlarged tragedy.

The United States has supplied humanitarian aid and diplomatic pressure. But our reluctance to arm the rebels or defend the civilians being slaughtered in their homes has convinced the Assad regime (and the world) that we are not serious. Our fear that arms supplied to the rebels would fall into the hands of jihadis has become a self-fulfilling prophecy, because instead of dealing directly with the rebels we left the arming to fundamentalist monarchies, Saudi Arabia and Qatar, and they are predictably using lethal aid to appease the more radical Islamists.

It might have been easier to intervene a year ago, before the opposition was so fragmented. But back then the president was busy ending foreign engagements and in no mood to start a new one. Besides, everyone was preoccupied by the dramas of Iran’s nuclear program, Egypt’s revolution and Ohio’s electoral votes. Since then, Assad has been sly about escalating his savagery by degrees — artillery, then aerial bombardment, then Scud missiles and now, apparently, chemical weapons — while staying just below whatever threshold of horror might shame us into responding.

What you hear from the Obama team is that we know way too little about the internal dynamics of Syria, so we can’t predict how an intervention will play out, except that there is no happy ending; that while the deaths of 70,000 Syrians are tragic, that’s what happens in a civil war; that no one in the opposition can be trusted; and, most important, that we have no vital national interest there. Obama conceded that the use of poison gas would raise the stakes, because we cannot let the world think we tolerate spraying civilians with nerve gas. But even there, the president says he would feel obliged to respond to “systematic” use of chemical weapons, as if something less — incremental use? sporadic use? — would be O.K. This sounds like a president looking for excuses to stand pat.

In contemplating Syria, it is useful to consider the ways it is not Iraq.

First, we have a genuine, imperiled national interest, not just a fabricated one. A failed Syria creates another haven for terrorists, a danger to neighbors who are all American allies, and the threat of metastasizing Sunni-Shiite sectarian war across a volatile and vital region. “We cannot tolerate a Somalia next door to Israel, Lebanon, Jordan, Iraq and Turkey,” said Vali Nasr, who since leaving the Obama foreign-policy team in 2011 has become one of its most incisive critics. Nor, he adds, can we afford to let the Iranians, the North Koreans and the Chinese conclude from our attitude that we are turning inward, becoming, as the title of Nasr’s new book puts it, “The Dispensable Nation.”