MOSUL, Iraq  A string of checkpoints has appeared on the roads that spoke out from this volatile city, guarded by hundreds of American soldiers working with Arab and Kurdish troops.

The joint operation along one of Iraq’s ethnic trouble spots began with a deliberate lack of fanfare, but it constitutes the most significant military mission by American forces since they largely retreated to bases outside Iraq’s cities in June.

More than two dozen checkpoints now punctuate a snaking line that traces  from Syria to Iran  the unofficial and disputed boundary between Iraq’s federal forces and those of the Kurdish regional government. At times these forces have operated virtually as opposing armies rather than as compatriots, but at the checkpoints they now live and operate together for the first time since the war began.

Guarding checkpoints  a task the American military never relishes  invites attacks by insurgents, who remain particularly active in northern Iraq. On three consecutive nights recently, rockets or mortars landed near three checkpoints in Diyala Province, though they caused no casualties, according to an American military spokesman and an Iraqi military official. “You stay static,” said First Sgt. Tony DelSardo of the Army’s Third Infantry Division, “you’ll get hit.”