The scenario now unfolding in Somalia is the one a U.S.-backed Ethiopian invasion nearly two years ago had been intended to thwart: a takeover by radical Islamists.

At the time, Ethiopian forces ousted a relatively diverse Islamic movement that had briefly gained control of the capital, Mogadishu. In its place, they installed a transitional government headed by a warlord who allowed the United States to launch counterterrorism operations in the moderate Muslim nation.

But the policy backfired, inspiring a relentless insurgency of clan militias and Islamist fighters that has left Somalia's first central government since 1991 near collapse.