The international offensive against the Islamist al-Shabaab organization in Somalia appeared to take a new turn last week as hundreds of Ethiopian troops accompanied by armored personnel carriers reportedly crossed the border into the famine- and war-torn nation.

The U.S. had already expanded its drone war into Somalia over the summer, and Kenyan troops have been fighting al-Shabaab in southern Somalia since last month. A significant incursion now by Ethiopia could shift the dynamics of the already chaotic war. ...