( CNN ) -- President Barack Obama faces a terrible choice in the coming days . It is whether to kill a number of Syrians who had nothing to do with a poison-gas massacre to show President Bashar al-Assad , who presumably did have something to do with it , that using chemicals weapons violates an international norm . Few would doubt the importance of international agreements to regulate war , particularly those that safeguard the rights of citizens , such as bans on chemical weapons and torture . If al-Assad is responsible for the use of chemical weapons , he had full knowledge of the norms he was violating . By way of comparison , President George W. Bush chose a policy that allowed torture in interrogations with full knowledge that it was outlawed by agreements to which the United States was a signatory . He made a strategic choice in the face of a threat to national security . Both are abhorrent actions on the part of national leaders , both taken for fundamentally the same reasons . An attack on the Syrian military by the United States would send a message to al-Assad , but it is not clear if Obama has any strategic objective beyond that . Whether he has such a plan or not , any U.S. attack will have implications for the civil war , no matter how much Obama says otherwise . If the United States launches missiles into Syria , those cheering the loudest will be the Syrian rebels . The United States will be intervening in the Syrian civil war on behalf of the rebels . A significant amount of research , including my own , demonstrates that military interventions from outside states lengthen and make bloodier civil wars . Much of this evidence is the result of statistical modeling of all civil wars and any associated interventions . The data include roughly 1,000 interventions into 100 civil wars over the last 60 years , with research carried out by multiple research teams . The results point to patterns in what happens when states intervene to try to help their preferred actor , and the results are strong and consistent that interventions rarely work to promote peace or reduce violence . For example , my own research has shown that the likelihood of a civil war lasting for four years without an intervention is 37 % , but if there is an intervention the likelihood that it lasts for four years is 60 % . The intervention accounts for the 50 % increase in the length of the war . Longer wars are generally bloodier . Examples might include the Nicaraguan war in the 1980s , and the Syrian war up to this point . The meager support for the rebels has allowed them to push harder against the government , and whether or not we support the government , the killing and the war 's duration has only increased . It 's hard to predict exactly what will happen , of course , but once the United States bombs Syrian forces , it is possible that the rebels will take that moment to launch their own offensive , forcing the Syrian government to respond even more resolutely , greatly worsening the bloodshed . Even if a U.S. missile strike reflects inadvertent help for the rebels , history does not point to a good outcome for U.S. policy . The only pathway by which external interventions consistently make for shorter or less bloody wars is through diplomatic efforts to broker a peace agreement . The same modeling evidence suggests that the war in Angola , 1975-1991 , could have been shortened by nearly 10 years with well-timed diplomatic interventions . From this perspective , a missile strike will only move the situation further from peace . Sometimes the difficulty for warring parties to come to a negotiating table is tied up in information that is held private , the very thing that mediators can manage . So why not engage in serious diplomacy ? The United States has two stated policies , at least beyond the immediate question of punitive bombing in response to the chemical attacks . The first is to aid the Syrian opposition in its fight against the Assad regime ; the second is to help broker a negotiated settlement . The second goal makes sense ; the first is fraught with difficulties . Two aspects of the negotiated route pose a problem for the United States . First , the official U.S. policy is that al-Assad must go . Both former Secretary of State Hillary Clinton and Secretary of State John Kerry have made these claims . Second , the rebels have steadfastly refused to negotiate . The only policy on the table is to kill more Syrians to show the Syrian leaders that killing Syrians is wrong . In this regard , U.S. policies are inconsistent with past behavior , empirical evidence and logic . The opinions expressed in this commentary are solely those of Patrick M. Regan .
Patrick M. Regan : Obama facing decision on striking Syria over use of chemical weapons . Whatever strategic reasons , he says , Obama 's act would have implications for Syria 's civil war . Comment: Regan : Statistical evidence suggests well-timed negotiations can shorten war .