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Washington • Confronted with evidence that chemical weapons have been used in Syria, President Barack Obama now finds himself in a geopolitical box, his credibility at stake with frustratingly few good options.

The origins of this dilemma can be traced in large part to a weekend in August, when alarming intelligence reports suggested the besieged Syrian government might be preparing to use chemical weapons. After months of keeping a distance from the conflict, Obama felt he had to become more directly engaged.

In a frenetic series of meetings, the White House devised a 48-hour plan to deter President Bashar Assad of Syria by using intermediaries like Russia and Iran to send a message that one official summarized as, "Are you crazy?" But when 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 declaredin 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. 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. U.S. 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 overnight Thursday, apparently targeting advanced missiles bound for the Shiite Lebanese militant group Hezbollah, highlighted the volatile situation.

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. Whether or how he chooses to follow through on his warning about chemical weapons could shape his remaining time in office.