The closer the US gets to its original goal in Syria of defeating the Islamic State group, the murkier its end game. The battlefield is shifting as demonstrated by a deadly barrage of American air and artillery strikes on a shadowy attacker.

The Pentagon insists it is keeping its focus on defeating IS, but Defense Secretary Jim Mattis said Thursday US-backed fighters in eastern Syria faced a “perplexing” overnight assault by about 300 pro-Syrian government fighters whose nationalities, motives and makeup he could not identify.

A number of US military advisers were present alongside local allied forces, and the Americans led a punishing response that other officials said killed about 100 of the assailants.