Last week my older daughter, who is studying Turkish in college, asked in an email whether I thought there would be a war between Syria and Turkey. I replied, "I don't think so, but it's possible," and then listed some reasons why war could happen. Sometime after clicking "send" I realized that those reasons were so powerful that I should have told her there's at least a fifty-fifty chance of war.
Before explaining what I mean, I should emphasize that I don't think either government really wants war. God knows Bashar Assad has his hands full fighting a civil war, and Turkish Prime Minister Recep Erdogan would presumably like to avoid war (particularly what Syria expert Joshua Landis has called the "potential Vietnam" that could result from putting ground troops in Syria). That Erdogan is scrambling to find alternatives to war is evident in his administration's pointedly suggesting this weekend that Syria's vice president would be acceptable as the leader of a transitional government.
But wars are often fought by countries whose leaders didn't really want them. (See World War I.) A common reason is that neither regime feels it can afford to be seen by its people as backing down. But perhaps more important are some other dynamics pushing these countries toward war:
 Turkey could decide before long that war is preferable to the alternatives. The Syrian civil war is creating all kinds of problems for Turkey. There's a big influx of refugees, and there's also the Kurdish issue: Many of Syria's Kurds hope to use the civil war as an opportunity to carve out an autonomous or even sovereign Kurdish region in Syria, and Turkey fears that this could prove contagious, emboldening Kurdish separatists in Turkey and energizing longstanding dreams of a new Kurdish nation that comprises parts of Turkey, Iraq, Syria, and Iran. Both of these issues--refugees and Kurdish nationalism--could lead Turkey to conclude that the sooner the Syrian civil war ends, the better. And helping fight it could help end it--especially if Turkey's fellow members of NATO help out.
 Speaking of NATO: The fact that a Turkish-Syrian war could draw America into the conflict will make such a war more attractive to some backers of American intervention. That includes some influential Americans (largely, but not entirely, drawn from the crowd that got the U.S. into the Iraq war), but it also includes non-Americans, among them, presumably, the leaders of some Arab states. And the more influential players there are who want a war to happen, the more likely it is to happen.
 Syria feels it can't afford to ignore the Turkish border. The casual reader of the news might ask: If Syria doesn't want a war, and Syrian shells that fall on the Turkish side of the border could start a war, why doesn't Syria quit firing shells anywhere near the border? Indeed, why not bow to the Turkish demand for a buffer zone on the Syrian side of the border? The answer is simple: The Syrian regime is fighting for its life, and along the Turkish-Syrian border lies the lifeline of its enemy. The rebels are being supplied with weapons via Turkey and are seizing control of border crossings inside Syria, and their goal is to build, from there, an expanding zone of control. Can you imagine any regime, in the Syrian regime's situation, not fighting to keep control, or retake control, of border crossings, and not trying to disrupt the enemy's supply of arms and ammunition near the point of origin?
 Turkey is, in a sense, already at war with the Syrian regime. The rebels aren't just being supplied via Turkey; they're being supplied by Turkey--at least in the sense that Turkey has willingly, even eagerly, turned itself into an arms and ammunition conveyer belt that is stocked by such countries as Saudi Arabia and Qatar. (The US is confining its contribution to "non-lethal" supplies such as communications equipment--which, of course, does in fact help the rebels kill people.) So the Turkish indignation at shells landing on Turkish soil is ironic; Turkey is sending lots more ammo into Syria than Syria is sending into Turkey--the difference is that the ammo Turkey is sending actually kills lots of people. That Turkey's role as a conduit of arms and ammunition isn't a passive one may make Syria even less inclined to keep its fire far from the border.
None of this changes the fact that the Syrian regime doesn't want a war with Turkey or the fact that the Turkish regime can't possibly be sanguine about war with Syria. But in the end these two facts may not matter.