BEIRUT, Lebanon — Not long ago, rebels on the outskirts of Damascus were peppering the city with mortar rounds, government soldiers were defecting in droves and reports circulated of new territory pried from the grip of President Bashar al-Assad.

As his losses grew, Mr. Assad unleashed fighter jets and SCUD missiles, intensifying fears that mounting desperation would push him to lash out with chemical weapons.

That momentum has now been reversed.

In recent weeks, rebel groups have been killing one another with increasing ferocity, losing ground on the battlefield and alienating the very citizens they say they want to liberate. At the same time, the United States and other Western powers that have called for Mr. Assad to step down have shown new reluctance to provide the rebels with badly needed weapons.

Although few expect that Mr. Assad can reassert his authority over the whole of Syria, even some of his staunchest enemies acknowledge that his position is stronger than it has been in months. His resilience suggests that he has carved out what amounts to a rump state in central Syria that is firmly backed by Russia, Iran and Hezbollah and that Mr. Assad and his supporters will probably continue to chip away at the splintered rebel movement.