Before using force in Syria, President Obama must articulate his intended political and military objectives, and explain how military force could plausibly achieve them. Policymakers and pundits routinely provide multiple objectives — but they tend omit the crucial second consideration. Consider some recent justifications offered for intervening in Syria’s ongoing civil war.

Prevent additional use of chemical weapons. Rep. Mike Rogers called for “action to disrupt [Assad’s] ability to deliver chemical weapons,” while Sen. Dianne Feinstein declared last week: “It is clear that ‘red lines’ have been crossed and action must be taken to prevent larger-scale use. Syria has the ability to kill tens of thousands with its chemical weapons.” The objective here is to deny Assad reliable access to one of his lethal military capabilities, reportedly used in small amounts, but to ignore the artillery, airstrikes, and sniper fire responsible for the vast number of civilian deaths. Some scholars and analysts also contend that a stronger U.S. response is mandated to maintain and reinforce the long-standing international taboo against the use of weapons of mass destruction.

The difficulty with preventing the use of chemical weapons, or securing and consolidating the several dozen sites where they are held, is that it is a resource-intensive military mission, requiring up to 75,000 troops. Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, warned in January: “The act of preventing the use of chemical weapons would be almost unachievable. You would have to have such clarity of intelligence, persistent surveillance — you’d have to actually see it before it happened. And that’s unlikely.” …