So those seeking to strike Syria must balance between a chemical convention that does not entail force and the responsibility to protect doctrine, which does, but “takes them away from their stated rationale,” he said.

Although President Obama has not yet made clear his own intentions, French and British officials suggest that an intensive missile attack on Syrian government and military installations and airfields, even of short duration, could not only degrade Syria’s ability to deploy chemical weapons but also encourage Mr. Assad to join in serious negotiations in Geneva to find a political solution to the long conflict.

They also note that there is likely to be considerable support from Sunni countries in the Middle East that see the civil war in Syria as a proxy for the struggle between Iran and themselves, and between themselves and Sunni Islamic radicalism.

“Because so many countries are lined up against the Syrian regime, the question is not how many countries will support the operation, but how many will be able to actively contribute to it,” Mr. Joshi said. “It was easier in Libya, with uncontested airspace. But given the short duration of such an action, none of this makes much difference from the diplomatic point of view.”

François Heisbourg, a defense expert at the Foundation for Strategic Research in Paris, said he was not particularly concerned about the complications.

“In strictly legal terms the case for this one won’t be great, but as in Kosovo, the law will have to fit the action,” he said. “But unlike Kosovo, this is not going to war versus Bashar or regime change. It’s a one-off strike, with older legal precedents in humanitarian interventions.”

As important, he said, is upholding the norm against the use of chemical weapons, which even the Russians have supported, endorsing Mr. Obama’s “red line” last year.