WASHINGTON — In the last days of November, Israel’s top military commanders called the Pentagon to discuss troubling intelligence that was showing up on satellite imagery: Syrian troops appeared to be mixing chemicals at two storage sites, probably the deadly nerve gas sarin, and filling dozens of 500-pounds bombs that could be loaded on airplanes.

Within hours President Obama was notified, and the alarm grew over the weekend, as the munitions were loaded onto vehicles near Syrian air bases. In briefings, administration officials were told that if Syria’s increasingly desperate president, Bashar al-Assad, ordered the weapons to be used, they could be airborne in less than two hours — too fast for the United States to act, in all likelihood.

What followed next, officials said, was a remarkable show of international cooperation over a civil war in which the United States, Arab states, Russia and China have almost never agreed on a common course of action.

The combination of a public warning by Mr. Obama and more sharply worded private messages sent to the Syrian leader and his military commanders through Russia and others, including Iraq, Turkey and possibly Jordan, stopped the chemical mixing and the bomb preparation. A week later Defense Secretary Leon E. Panetta said the worst fears were over — for the time being.