As the Obama administration scrambles to counter the Islamic State, commanders have decided against trying to rebuild entire vanished divisions or introduce new personnel in underperforming, undermanned units across the country, according to U.S. officials. Rather, the officials said, the hope is to build nine new Iraqi army brigades — up to 45,000 light-infantry soldiers — into a vanguard force that, together with Kurdish and Shiite fighters, can shatter the Islamic State’s grip on a third of the country…

The development of a spearhead force is unlikely to address the larger decay across Iraq’s security forces and institutions, a more complex, deeply rooted phenomenon that undermines the country’s stability. The force is also insufficient on its own to retake strategic cities such as Mosul.

But U.S. officials and others said the training of a smaller number of high-quality units could enable Iraqi security forces to make significant headway against the Islamic State — supplemented eventually, U.S. officials hope, by a new “national guard” that could bring an array of armed groups operating across Iraq under provincial government control.