War drums are beating once again in Washington.

The Syrian civil war has left 70,000 dead over the last two years. Roughly 1.3 million people have been driven from their homes, with some living in caves to escape the shelling. The stories are horrifying. No one with a conscience can fail to be moved by the suffering of the Syrian people or outraged by the actions of Bashar al-Assad's dictatorial regime in Damascus. But that doesn't mean that calls for the U.S. to intervene militarily make sense. On the contrary, such suggestions are profoundly foolish. 

The case for intervention has been weak from the beginning, and it's been getting steadily weaker. Yes, the rebels oppose the execrable Assad regime, but many of them also appear to be jihadists. Would a post-Assad Syria be more free or less free than it is now? More democratic or less democratic? Better for women or worse for women? No one knows. Recent press reports indicating that at least some of the rebels are allied with al Qaeda certainly don't inspire high hopes. (Sometimes the enemy of our enemy is an even worse enemy.) Then there's the fact that Russia and China continue to oppose any action to topple the Syrian dictator, and flagrantly antagonizing them could make U.S. policymaking much more complicated in other parts of the globe.

Why, then, are the interventionists gaining traction now? Because last August President Obama warned the Syrian dictator that he considered the use of chemical weapons in the conflict a "red line" — and it now appears that Assad has crossed it

On this the interventionists are right: When the president makes a threat, he has to follow through on it — otherwise, our ability to deter Iran, North Korea, and China (in the Taiwan Strait) will be severely undermined. Which means that Obama may be about to get us mixed up in yet another war in the Middle East.

This may be his gravest foreign policy blunder to date.

The president's decision to lay down his red line on chemical weapons was based on two errors in judgment — one large, and one small. 

The first error is hardly unique to Obama; in fact, it's made constantly in the nation's capital. When atrocities are committed, when innocents suffer, when cities are shelled, when citizens are turned into refugees and children into orphans, the instinctual response among Washington politicians, journalists, and policy wonks is to assume we need to "do something" to stop it. While the instinct is admirable, it is based on the unexamined assumption that the United States has the same moral duties as an individual human being. It does not.

If a strong and well-armed individual refuses to come to the aid of someone being assaulted, we judge that person harshly — because his obligations are clear: He should defend the victim, even at the risk of injury to himself. If he displays a willingness to sacrifice his own well being in the act of fulfilling his moral duty, we call him selflessly courageous; it he doesn't, we denounce him for cowardice and selfishness. That's how moral judgment works.

But it's not how statesmanship works. The primary duty of the nation's commander in chief — the duty that overrides all others — is to uphold the common good of the United States and protect the rights of individual American citizens. If that sounds selfish, that's because it is. And rightly so. The president's duty is to us. He can have no duty to the citizens of another nation. That's why the greatest acts of statesmanship will always be more self-interested than the highest acts of individual virtue.

In keeping the United States out of the Syrian conflict for the past two years, Obama has showed that he understands this. But in laying down his now-transgressed "red line" on chemical weapons, he showed that he doesn't understand it well enough. It's as if the president wants to have it both ways: to be a tough-minded realist who puts American interests first, but also to become an idealistic do-gooder (who, like all presidents, salves his conscience by ordering other people — the nation's soldiers — to sacrifice themselves) once a certain line has been crossed. And Assad has called Obama's bluff.

Did Obama at least draw his moral line in a sensible place? Unfortunately, we have reason to doubt that, too — and therein lies his second mistake. Chemical weapons are very bad, of course. But are they categorically worse than aerial bombardment of civilian targets using conventional weapons? Both, after all, produce piles of corpses and leave large numbers of victims maimed and disfigured. The end result in both cases is horrifying. Should one provoke a military response while the other does not? If Obama thinks so, he has yet to make the case.

None of this means we ought to be propping up Assad, as Russia, China, and Iran are doing. But it does mean that the president should have been content to keep out of the conflict, as 62 percent of the American people seem inclined to do even now.

America has neither the will nor the resources — nor the moral obligation — to act as the world's policeman. Or as its instrument of salvation. Until the powers that be in Washington learn that crucial lesson, U.S. foreign policy will remain poised somewhere between folly and farce.