The debate over whether or not to intervene in Syria draws on two logics that portend inaction. First, political scientists claim that external meddling in internal conflicts only leads to protracted wars -- interventions lengthen, not reduce, conflicts. Second, to actually arm Syria's rebels would require armaments such as manpads and other hardware that could be used against Israel were an Islamist government to assume power in a post-Assad Syria, not unlike how the Afghan mujahedeen turned against the United States after the Soviet invasion.
Let's dissect the above. First, scholars find that interventions, especially those on the side of rebels, do prolong civil wars. But the evidence, drawn mostly from the Correlates of War (COW) dataset, may be heavily skewed because of a few protracted conflicts -- namely civil wars in places like Angola that saw no shortage of external involvement -- and also suffers from selection issues. For instance, it is possible that outside powers select into civil wars because they are protracted, not that the act of arming one side makes war last longer (Page Fortna points to similar selection issues to explain "successful" outcomes of UN peacekeeping missions). Indeed, it is unclear why tipping the balance of power toward one side would make war last longer. The supply of arms and other supplies is less studied. In at least one case -- the French intervention against the British during the American Revolutionary War -- the supply of arms after the Battle of Saratoga definitely tipped the balance of power in the Yankees' favor (of course, it helped that the French sent their navy too). Also, Iraqi insurgents enjoyed very little external support yet were arguably able to fight the United States to a long (ten years and counting) and ugly stalemate (We also know that unarguably the presence of a NATO no-fly-zone drastically reduced what would have been a drawn-out civil war in Libya). In other words, not arming the Syrian rebels does not mean the war will end any sooner, it only means we will have no skin in the game in a post-Assad Syria.
In Never Ending Wars (2005), Ann Hironaka argues that the spate of outside interventions was a function of the spread of weaker states after World War II which were unable to control their borders. Nearly three out of four civil wars since 1945 have experienced third-party intervention, most of which have entailed the supply of arms, aid, and bases, not putting boots on the ground. Of the 49 conflicts with no third-party intervention, the average length of conflict was 1.5 years. By contrast, those with outside intervention saw an average length of seven years. However, these conflicts were protracted largely because outsiders were intervening on both sides (external intervention by major powers during the 19th century was more one-sided and thus actually made such crises shorter, not longer).
Indeed, I would argue that the literature on external interventions is not entirely undivided. Nile Metternich, for example, finds that interventions by international organizations (e.g. NATO), especially those with democratization mandates, are associated with shorter conflicts, provided rebel leaders come from ethnic groups representing more than 10 percent of a country's population (which would fit Syria's largely Sunni opposition). Clayton Thyne looks at unobserved variables (e.g. high levels of resolve among the combatants) that contribute to the resolution of conflicts even with third-party interventions.
The notion that somehow arming Syria's opposition means a long and protracted war is misguided and driven by a realist-inspired desire to stay on the sidelines and never intervene anywhere. Several foreign policy experts chalk it up to a political gesture and a way to check the box that we are merely doing something. That is bunk. Arming the opposition would send a clear signal to the Assad regime that the president's words are not merely rhetoric, that there is a responsibility to protect citizens and refugees, and that we can tip the balance of power in the rebels' favor. It would also help wean the influence of Islamist actors in the region, since many of the arms flowing into their hands are coming from Qatar and other places that favor Islamist over secular groups.
On the second point, would arming the opposition have blowback effects against allies such as Israel after the war ends? This is obviously a concern but it should not lead to a policy of paralysis. Israel, after all, has responded to attacks from the Assad regime and has even provided military intelligence related to alleged chemical weapons use in Syria. We should no doubt screen which factions within the opposition receive light weaponry, but more importantly, we should be clear that the opposition is not predisposed to turn against the United States or Israel should Assad fall in the near future. That can be done through providing them with assistance -- intelligence, training (which we are doing in Jordan), funds, etc. -- contingent on future behavior. A stronger opposition will face less chance of a reconstituted Alawite faction trying to wrestle control of a Sunni-led regime (albeit the risk is that we may be arming them to fight the Alawites should Assad fall, a risk worth taking).
Not arming the rebels both scratches our realism itch to not involve ourselves in messy internal wars, as well as not arm a potentially hostile future Syria that would pose a threat to Israeli citizens. The second scenario is real, but the first should be discounted: doing nothing (or at the least, giving the rebels non-lethal equipment and some humanitarian aid) is not an option. The U.S. will get pulled into the conflict, either on our terms and our timeline, or on terms we do not foresee (consider a Syrian strike against Israel, which would draw us in). To be sure, by arming the opposition we will be taking sides in a seriously bloody civil war. But this is not Vietnam or Afghanistan in the making. Arguably Syria matters more than these two states to their respective regions' balance of power. As Syria goes, so goes the Middle East. A victory by Assad cements the continuing influence of Iran and Hezbollah to stir up trouble in the region, from nuclear proliferation to terrorist attacks. The removal of Assad at least provides an alternative, albeit less predictable, path forward for the region. Yes, Islamists would likely wield more power in a post-Assad Syria but that is not guaranteed to spell deteriorating Israeli-Syrian relations or war. Israel still possesses a powerful nuclear deterrent and the backing of the U.S. The worst possible scenario for Israel is a worsening civil war on its doorstep, not an Islamist in power in Damascus.
The fate of Syria will reshape the Middle East for generations. The Obama administration's defeatist attitude and deer-in-headlights policy will only prolong the conflict, not hasten its successful conclusion.