MOSCOW — Secretary of State John Kerry had just made an offhand remark about how President Bashar al-Assad of Syria could avoid a military strike — and now Sergey V. Lavrov, Russia’s hard-charging foreign minister, was on the phone.

Mr. Lavrov was not about to let the moment pass.

What aides to Mr. Kerry were already trying to roll back, Mr. Lavrov seized on, telling Mr. Kerry he would immediately go public with a Russian-led proposal to dismantle the Syrian chemical weapons arsenal. That prompted a sharp response from Mr. Kerry who warned in the 14-minute call, “We are not going to play games.”

By the time Mr. Kerry’s plane landed back in Washington, the ground had shifted and on Saturday, not a week later, Mr. Kerry and Mr. Lavrov completed the plan sitting by the pool at a Geneva hotel.

It is a pact that American arms control experts have scrambled to shape and that the White House believes may be the best way to the uphold prohibitions against the use of poison gas without resorting to military force. But it is also one that the Kremlin clearly thinks serves the interests of Russia and the Syrian government.