Members of Congress and the foreign-policy establishment are warning that a U.S. intervention in Syria could backfire and lead to greater civilian casualties by prompting an intensification of the civil war there and a crackdown by the Assad regime on its own people. Democrat Chris Murphy, who sits on the Senate Foreign Relations Committee, told the L.A. Times he worried that a U.S. intervention that amounted to "little more than a slap on the wrist" could "mean an even greater loss of life within Syria" down the road.
It's a risk the administration is aware of. “There’s a possibility that the Syrian government would use chemical weapons again, and I don’t think you can discount that,” a senior administration official told Defense One. “You’ve got to remember, this is a government, a regime, essentially a dictatorship that is playing for its survival, and when you’ve got a situation like that, then these people use any means they can to survive.”
A 2012 study by Reed Wood, Jacob Kathman, and Stephen Gent published in the Journal of Peace Research gives credence to those worries. "Military interventions in favor of the rebel faction (as opposed to pro-government or neutral interventions) tend to increase government killings of civilians by about 40 percent," wrote Professor Erica Chenoweth of the Josef Korbel School of International Studies at the University of Denver over at the academic blog The Monkey Cage, summarizing the research. By upsetting the balance of power in intrastate conflicts, outside interventions can cause a regime that sees its hold on power weakening to redouble its efforts and lash out brutally.
From Wood, Kathman, and Gents's abstract:
As a conflict actor weakens relative to its adversary, it employs increasingly violent tactics toward the civilian population as a means of reshaping the strategic landscape to its benefit. The reason for this is twofold. First, declining capabilities increase resource needs at the moment that extractive capacity is in decline. Second, declining capabilities inhibit control and policing, making less violent means of defection deterrence more difficult. As both resource extraction difficulties and internal threats increase, actors’ incentives for violence against the population increase. To the extent that biased military interventions shift the balance of power between conflict actors, we argue that they alter actor incentives to victimize civilians. ... We test these arguments using data on civilian casualties and armed intervention in intrastate conflicts from 1989 to 2005. Our results support our expectations, suggesting that interventions shift the power balance and affect the levels of violence employed by combatants.
You can read their complete study here.