Dhaka, Sept. 20 -- The Houthis began as a religious revival movement in Yemen's rugged northern highlands. Three decades later, they control the capital, possess ballistic missiles and long-range drones and have demonstrated an ability to disrupt one of the world's most important shipping routes.

Their latest battlefield gains have underlined just how far that transformation has gone.

In September, Houthi forces swept through Yemen's southern Red Sea coast, taking the port of Mocha and the remaining coastline and islands held by their opponents. The advance put their forces along the Yemeni shore of Bab el-Mandeb, the 19-km-wide strait linking the Red Sea with the Gulf of Aden.

Bab el-Mandeb is a major chokepoint on the shipping route ...