While Aleppo is the proximate prize sought by the government and its Russian backers, at least 50,000 opposition fighters — many of whom owe their training, weapons and inspiration in large part to the United States — remain in pockets spread across western Syria. Many of those forces have been advised and supplied by the CIA, whose director, John Brennan, is said to favor military action or, at the very least, dispatching more and better weapons to the opposition, particularly if Aleppo is lost. That decision, which would allow the rebels to continue to fight a guerrilla war, or to defend those pockets of the country still in opposition hands, might not be the administration’s to make. Allied governments in the region, including Qatar, Turkey and, to a lesser extent, Saudi Arabia, have long advocated for increased support for the rebels and could decide on their own to send more sophisticated armaments — some of which, including shoulder-launched antiaircraft weapons, the United States has refused to make available on the grounds that they could end up in the wrong hands.