The odds of a U.S. military response either directly against Iran or against the militias in Iraq backed by Tehran have risen following a Dec. 27 rocket attack on a military base in Kirkuk that left a U.S. contractor dead and four U.S. military personnel injured. At least 30 rockets struck the K-1 base in northern Iraq that houses both U.S. and Iraqi military forces. Both Islamic State militants and Iranian-backed militias known as Popular Mobilization Units (PMUs) have been active in the area, and both have conducted attacks around the base. But the scale of the Dec. 27 attack and the types of rockets launched make PMUs a more likely suspect in the assault than the Islamic State. In fact, U.S. officials have singled out Iranian-backed Kataib Hezbollah as a possible perpetrator....