President Obama’s new strategy for routing ISIS, the extremist Sunni group that controls large areas of Iraq and Syria, rests substantially and precariously on having rebels in Syria fight ISIS, even as they battle the forces of the Syrian president, Bashar al-Assad. The plan is full of hope and fraught with obstacles.

During the three-year-long Syrian civil war, Mr. Obama has been rightly reluctant to provide significant weapons and military assistance to the Syrian rebels. From the beginning, it was nearly impossible to determine the makeup and character of the rebel groups, of which there are about 1,500, according to James Clapper Jr., the director of national intelligence.

Groups identified by Western intelligence agencies as the moderate opposition — those that might support democracy and respect human rights — have been weak, divided and without coherent plans or sustained command structures capable of toppling the Assad regime. Today, those so-called moderates are even weaker and more divided; in some cases, their best fighters are hard-line Islamists.

In ruling out sending American combat troops into yet another Muslim country, Mr. Obama’s plan relies on these rebels to serve as ground forces to defend and seize territory after American airstrikes in Syria, for which he needs to seek congressional approval. But training and equipping them will be complicated and risky, and will take months, if not longer. ISIS, which the C.I.A. said Thursday has as many as 31,500 fighters in Iraq and Syria, is already well-equipped and has proved to be stunningly skillful at waging war and seizing territory in both Iraq and Syria.