Even while Israel appeared to be egging on Mr. Obama toward taking action, with officials here saying Tuesday that it appeared sarin gas had been used by the Syrian government, those officials also conceded that none of the military options were good.

“If you bomb the sites, you could cause exactly the catastrophe you are trying to prevent,” said an Israeli military officer who has spent considerable time studying the options. “If you just go in to secure the weapons, you can get stuck” in the middle of a civil war, he added, with American troops and their allies suddenly targets, and no easy way out…

Iran, too, may well be watching Mr. Obama’s decision-making on Syria closely. Ayatollah Ali Khamenei has huge stakes in the survival of Mr. Assad, his only real ally in the region. And United States intelligence analysts believe that Iran’s leaders have interpreted two decades of American drift on the North — during which Mr. Obama’s three immediate predecessors all said they would never tolerate the country’s obtaining nuclear arms — as a sign that Washington will not wage war to stop even a rogue nation from obtaining nuclear arms, or the ability to build them.

If the United States intervened in Syria to secure its chemical stockpiles — perhaps organizing the Arab League, the members of the Gulf Cooperation Council or NATO to share the job — Israeli officials say it would be a signal that Mr. Obama would most likely back up his warnings to Iran the same way.