Full-scale civil war in Syria seems unlikely on the surface simply because the protesters have no arms, but an endless stream of refugees will eventually prompt Turkey to do something, and whatever Turkey does will raise the likelihood of significant arms flowing into the arms of dissidents, increasing willingness of dissidents to fight back with force, and Syrian regime resistance with force. Once civil war occurs with Turkish forces inside Syria, Iran, Israel, and Saudi Arabia will start recalculating their options with increasing urgency.
From this point to a Spanish Civil War scenario, in which the powers exploit chaos in Syria to fight their broader fights, could be a short step. So the question becomes:
How likely is the Spanish Civil War Scenario in Syria, and what are the key decision points impacting the likelihood of such a scenario?
Turkey may well be able to intervene without greatly alarming any other actor. If either Iran or Israel were to insert significant military force, in contrast, that would immediately alarm the other, for good reason. The strategic difference between Syria as an Iranian ally and Syria as an Israel ally is substantial. How desperate to shore up their access to regional influence might Iran become?
How desperate to end Iranian influence over Syria might Israel become?
Calculating national interest concerning Syria is not simple. One could argue that, as status quo powers, the U.S., Israel, and Turkey would all benefit from a joint operation to eliminate the Assad regime. But if this operation left Israel effectively doing to Syria what it did in the 1980's to Lebanon, that would have an enormously destabilizing impact, greatly facilitating an Israeli attack on Iran and thus probably provoking risky Iranian countermeasures. Such destabilization is unlikely to be viewed with equanimity in Ankara. The case has already been made that the U.S. and Israel are trying to overthrow Assad. Whether or not literally true today, the temptation to pursue this old dream is clear...and rising. Will Ankara see this line of reasoning as evidence for a rapid unilateral intervention?
While even senior Israeli officials are sufficiently concerned about a miscalculation in Tel Aviv to express their fears publicly, it seems somewhat alarmist to anticipate Israeli aggression against Iran the minute they get the ability to base planes in Syria. In fact, any major military initiative designed to transform Syria into a proxy state by any outside player seems alarmist at the moment. The more likely route to a Spanish Civil War Scenario for Syria is a long series of short, seemingly harmless little steps in a complex dance in which no outside player actually wants to invade but in which each player feels compelled to match all the others. Politics being politics, the end result will no doubt be that most of the steps will more than match the opponent, like a group of waltzing couples on a slanted dance floor, each of whom is simply trying to keep up with some other couple perceived to be dancing faster; the faster they dance, the harder it is to stay in place, so imperceptibly the whole group moves closer and closer to the edge.
Given Israel's proclivity for overreaction, any step by Iran in the direction of stimulating a Syrian Hezbollah in the context of a collapsing Assad regime would make it very difficult for Tel Aviv to resist military intervention. A second way disaster could occur would be the rise of a serious Saudi-financed Salafi move to transform Syria into the center of Sunni activism in the region. This would strengthen Saudi claims to Muslim world leadership, constitute a direct defeat of Iran, and serve the Saudi campaign to pull Iraq away from Iran, while pushing the ambitious Turks back to the regional periphery. Once again, Israel would no doubt view this with alarm, though how it would balance off a check on Iran vs. an improvement in jihadi prospects is unclear since jihadis have not focused on targetting Israel. In any case, it is not hard to imagine a Spanish Civil War scenario leading quickly to a Greater Israel Scenario.