What if a US strike not only fails to deter the Assad regime from future use of chemical weapons, but also makes things worse, such as provoking attacks on Israel or Turkey or empowering extremist elements of the Syrian opposition more hostile to US interests than the current regime?
If the US were to launch strikes against Syria, Assad could respond with reprisals against his own people or attacks against US allies in the region. Allies such as Iran or Hezbollah also could launch their own reprisals, say lawmakers on Capitol Hill, citing the responses they're getting from constituents.
"What happens if this thing gets away from us?" said Sen. Jim Risch (R) of Idaho, noting the prospect that Hezbollah in Lebanon could launch a counterstrike against Israel. "What happens if they get into it with Israel? What's our response to that going to be?"
The president and top administration spokesmen insist that, even in such a case, the US has options and can respond effectively without going to war.
"If [Assad] is foolish enough to respond to the world's enforcement against his criminal activity, if he does, he will invite something fare worse, and I believe, something absolutely unsustainable for him," said Secretary of State John Kerry at a Sept. 3 hearing. "That doesn't mean the United States of America is going to war, as I said in my comments. There are plenty of options here."
In response, Sen. Chris Murphy (D) of Connecticut credited the Obama administration with having made a convincing case that atrocities had been committed in Syria but questioned whether Americans still believed that US military power could lessen that "moral atrocity" or advance US national interests in the region.
"And clearly, though there is not some direct linkage between what happened in Iraq and what happened in Syria, it does chill the ability of people to believe that American military might can influence politics on the ground in Syria after they have watched the last 10 years," he added.
Members of Congress say that their e-mail and letters, citing such concerns, are running strongly against the war.
"We get calls by the thousands: Nobody's calling in favor of this war," says Sen. Rand Paul (R) of Kentucky, a likely 2016 presidential contender. "We all agree that chemical attacks are a horrendous thing. But people are not excited about getting involved, and they also don't think it's going to work. And they're skeptical of what will occur with this."
On Sept. 5, the State Department issued a travel alert to Iraq, noting that terrorist activity is "at levels unseen since 2008." On Sept. 6, Turkey and Lebanon were added to the travel alert list.