Working from this sun-scorched desert base, U.S. and allied commanders are beginning perhaps the most perilous phase of their fight against Islamic State: an attempt to recapture Mosul, Iraq’s second-largest city, from the entrenched militant forces.

Military officers here say a barrage of airstrikes over the last two weeks helped sever two crucial routes that the extremist militants used to funnel fighters and supplies from the Syrian border to Mosul, their self-declared capital in Iraq and most significant battlefield prize.

U.S. commanders who help oversee the air war say the joint offensive with Iraqi Kurdish ground forces pushed back the Sunni Islamists’ defensive line west of Mosul, recapturing territory and removing a key obstacle, at least for now, as military planners consider tactics for retaking the congested city as early as this summer. […]