Sunni Muslim tribesmen, Shiite militia fighters and Iraqi security forces set out Saturday to recapture a key city in Anbar province and stop Islamic State atrocities against a local tribe in an extraordinary coalition that could stir sectarian tensions or serve as a model for future cooperation against the militants.

The operation to liberate Hit, about 90 miles west of Baghdad, could reshape the situation in Anbar in a way that would affect the mission of U.S. troops who are being deployed to the province from among the additional 1,500 U.S. military advisers the Pentagon said it is sending to Iraq at year-end.

“This is a dramatic change,” said Hisham al Hashimi, an Iraqi defense analyst. “We have the Sunni Arab tribes fighting hand in hand with the Shiites.” […]