Weeks after the Obama administration canceled a failed Pentagon program to train and arm Syrian rebels to combat the Islamic State, American officials announced a new effort to equip ground forces in Syria to fight the jihadists.

But 10 days of interviews and front-line visits across northern Syria with many of the forces in the alliance, called the Syrian Democratic Forces, made clear that so far it exists in name only, and that the political and logistical challenges it faces are daunting.

One Arab commander, sitting near the earthen wall that separates this deserted town in Syria from the Islamic State‚Äôs front line, bitterly recalled being chased from his Syrian hometown by the jihadists and said he would do anything to reclaim that territory. But then he detailed a list of things his forces needed: ammunition, radios, heavy weapons and more American airstrikes. […]