According to the polls, a little more than a majority of Americans oppose intervention in Syria, although it is difficult to say exactly what this means since the subject is decidedly ambiguous. Does intervention mean the sort of limited air campaign that President Obama seems to have in mind, or does it mean "boots on the ground" and indefinite occupation? Not an easy question to answer.

It is disingenuous to say that intervention is out of the question since the American people are "weary of war." Of course they are weary of war; most human beings grow weary of war fairly quickly. But Americans were undoubtedly weary of war 70 years ago, in 1943, or 150 years ago, in 1863, and with tens of thousands of good reasons. The difference is that, in both instances, they were guided by presidential leadership, and some semblance of coherence in making the case for war. By starting and stopping and starting again, above all by issuing declarations on one day and repudiating them the next, President Obama has failed an essential test of his office in the Syrian crisis. This has weakened Obama's authority; more important, it has diminished American leadership in the world.

It might have been possible, at some point early in the Syrian uprising, for the United States to intervene decisively: The Obama administration announced that Bashir Assad had lost "legitimacy" and had to go, and Obama and his then-secretary of state, Hillary Clinton, issued a series of warnings and red lines and ultimatums which could have been followed by punitive measures. But those circumstances obtain no longer: The Assad regime is now so deeply entrenched in Damascus that it will do anything to retain power -- use chemical weapons on noncombatant Syrians, for example -- and what began as a popular revolt against an autocratic regime has now deteriorated into a civil war of irregular armies of Islamists, tribal loyalists, Syrian conscripts, Iranian proxies, jihadists, uniformed Ba'aathists, and devotees of the Arab Spring.

The American interest in choosing sides is, admittedly, difficult to identify. But intervention is necessary for one good reason with historical resonance: American credibility. This is no trivial matter. As the British Parliament demonstrated last week, there is no will in the world to protect human rights, or to enforce what passes for international law, in the absence of American leadership. If President Putin in Moscow and President Xi in Beijing, not to mention President Assad in Damascus, recognize that the United States is retreating from its global commitments, they will hasten to fill the vacuum. This will not absolve the United States and its war-weary people from responsibility, and afford us a well-earned rest; it will make the world a more dangerous place, less stable, surely more perilous for democracy and free peoples. In practical terms, we cannot allow Bashir Assad to deploy chemical weapons, and promote regional chaos, without paying a price in authority, credibility, and above all, safety.

I say this, incidentally, as one with mixed feelings about the practical consequences of the Syrian conflict. Relatives of my paternal grandmother, fleeing from the Armenian genocide, managed to cross the Anatolian wasteland on foot at the end of World War I, and landed, along with thousands of other Armenians, in Aleppo, across the border from Turkey in northwest Syria. There they remained, and prospered -- until now. Because the Ba'ath Party in Syria is secular, and the Assads (as Alawites) are members of a religious minority, Christians have been comparatively unmolested in Syria since Armenians sought refuge there a century ago. No longer. Whatever the consequences of this civil war, there will be no protection for Aleppo's Christians from distant Damascus, and Islamist rebels are committed to ending Christianity's 2,000-year-old presence in Syria. I have no idea what has happened to these distant relations, and can only hope they have found refuge elsewhere.

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