A U.S.-backed effort to arm the moderate Syrian opposition is finally ramping up along the Turkey-Syria border, but it may come too late to save the rebels from defeats on two fronts, by President Bashar al-Assad’s government and by the extremists seeking to carve out an Islamic state.

Spurred by concerns that the al-Qaeda-inspired radicals will continue their relentless march across Iraq and Syria, the United States and its allies have begun accelerating the supply of arms and ammunition to a small number of vetted rebel groups in northern Syria, according to diplomats and rebels who have been receiving the deliveries.

Yet even as the fresh support arrives, challenges are mounting for the embattled moderates, who have been pushed out of eastern Syria by extremists, are being encircled in Aleppo by the government and are seeing their ranks eroded by defeats, desertions and infighting. […]