AfriCom joins the five other geographic commands of the U.S. military and is an acknowledgment of the growing strategic importance of the continent. For now, the command will be headquartered in Stuttgart, Germany, rather than in any African country -- a decision that itself speaks volumes about the region's instability. The U.S. already has a base in Djibouti, where a force of about 1,500 is poised to respond rapidly to crises in the Horn of Africa.

The new command will work with African nations to achieve -- as its explanatory material puts it -- "a more stable environment in which political and economic growth can take place." Toward that nonmilitary end, and unlike other commands, the four-star Army general at the top of AfriCom has a civilian deputy from the State Department. AfriCom will also provide military training to African countries and support the African Union's standby force.

But its most important mission is helping discourage and defeat terrorism on the continent. The Islamic insurgency in Somalia is one challenge; the U.S. warships currently surrounding the hijacked Ukrainian ship are there to make sure the arms it's carrying don't reach the militants. Another challenge is al Qaeda's North Africa wing, which is responsible for bombings in several countries in the past two years. These are all reasons to welcome AfriCom.

At the same time, a cautionary word is due: While the geographic commands are a useful tool for dividing up the Pentagon's work, the danger is that they are also the enemy of strategic thinking. Terrorists and other bad actors don't respect these artificial divisions. The Defense Secretary's leadership challenge is to encourage his regional commanders to think beyond the borders assigned to them.