When Iranian-backed militias repeatedly targeted U.S. troops in Syria and Iraq this fall, the Biden administration struck back with force. Action was needed, officials said, to deter the groups from turning Israel’s conflict with Hamas into a wider war.

But the United States has not yet retaliated against one Iranian-backed group: the Houthis of Yemen.

In the past month alone, the Houthis have launched more than 100 attacks against commercial vessels in the Red Sea, crippling traffic there.

So why has the United States taken a different approach with the Houthis? The reasons are many.

The Houthis have launched missiles and drones at vessels in the Red Sea and seized an Israeli-linked ship during more than two months of war between Israel and Hamas.