In a frenetic series of meetings, the White House devised a 48-hour plan to deter President Bashar al-Assad of Syria by using intermediaries like Russia and Iran to send a message that one official summarized as, “Are you crazy?” But when Mr. Obama emerged to issue the public version of the warning, he went further than many aides realized he would.

Moving or using large quantities of chemical weapons would cross a “red line” and “change my calculus,” the president declared in response to a question at a news conference, to the surprise of some of the advisers who had attended the weekend meetings and wondered where the “red line” came from. With such an evocative phrase, the president had defined his policy in a way some advisers wish they could take back.

“The idea was to put a chill into the Assad regime without actually trapping the president into any predetermined action,” said one senior official, who, like others, discussed the internal debate on the condition of anonymity. But “what the president said in August was unscripted,” another official said. Mr. Obama was thinking of a chemical attack that would cause mass fatalities, not relatively small-scale episodes like those now being investigated, except the “nuance got completely dropped.”