The war between Hamas and Israel in Gaza has over the past month made clear that there is a potential for a regional conflagration. Lebanese Hezbollah and Israel have repeatedly engaged in tit-for-tat clashes, and Yemen’s Ansarullah movement—better known as the Houthis—has fired missiles at Israel and seized an Israeli-linked vessel.

But it is in Iraq where the prospect of a broader confrontation involving US forces is perhaps most real. Scores of drone and rocket attacks against American military installations have taken place in recent weeks. While there has been a lull in such incidents in recent days, several important developments have taken place in Iraq following the Palestinian surprise attack on Israel in early October. There is an emerging split among Iraqi Shiite armed groups, the prime minister they have supported is finding himself stuck between a rock and a hard place, and some of...