“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.”

As a result, the president seems to be moving closer to providing lethal assistance to the Syrian rebels, even though he rejected such a policy just months ago. American officials have even discussed with European allies the prospect of airstrikes to take out Syrian air defenses, airplanes and missile delivery systems, if government use of chemical weapons is confirmed.

An Israeli airstrike in Syria on Thursday, apparently targeting advanced missiles bound for the Shiite Lebanese militant group Hezbollah, highlighted the volatile situation. With Syrians already dying by the thousands from conventional weapons, Mr. Obama now confronts the most urgent foreign policy issue of his second term, one in which he must weigh humanitarian impulses against the risk to American lives. After about two years of ineffectual diplomacy, whether or how he chooses to follow through on his warning about chemical weapons could shape his remaining time in office.

The evolution of the “red line” and the nine months that followed underscore the improvisational nature of Mr. Obama’s approach to one of the most vexing crises in the world, all the more striking for a president who relishes precision. Palpably reluctant to become entangled in another war in the Middle East, and well aware that most Americans oppose military action, the president has deliberately not explained what his “red line” actually is or how it would change his calculus.

“I’m not convinced it was thought through,” said Barry Pavel, a former defense policy adviser to Mr. Obama who is now at the Atlantic Council. “I’m worried about the broader damage to U.S. credibility if we make a statement and then come back with lawyerly language to get around it.”