The strategy’s biggest weakness in Iraq, officials there say, is the glacial pace of cobbling together an Iraqi political alliance between Sunnis willing to join with the Shiite-controlled central government to rebuild a national military force to fight Islamic State more effectively.

Key to that effort is getting Sunni tribal and political leaders and the Shiite political leaders to agree on a new initiative to build a new Sunni-staffed national guard to defend the central Iraqi front, including Baghdad, against further Islamic State advances.

U.S. officials have been on a lobbying blitz for most of the last two weeks to sign up major Iraqi Sunni tribal figures to the guard units and pressure the country’s new Shiite prime minister to deliver votes from his constituency, which hold a majority in parliament but who are deeply wary about the political allegiances of the Sunni leaders.

But the initiative has been shelved over a holiday week, and it remains unclear when the parliament might take it up.