Middle East

Expanding U.S. role in Iraq strains awkward alliance with Iran

The expanding U.S. military campaign against the Islamic State group in Iraq relies in part on an uneasy, arms-length partnership with Shiite militias backed by Iran — organizations that were once relentlessly effective killers of U.S. troops.

Now, as the campaign enters its second year, there are signs that this awkward alliance may be fraying: militia threats of renewed attacks on U.S. personnel, a greater U.S. effort to bolster Sunni forces that are traditional adversaries of Iran and accusations that the U.S. air campaign has at times targeted Shiite forces.