In the fall of 2003, the new commander of American forces in Afghanistan, Lt. Gen. David W. Barno, decided on a new strategy. Known as counterinsurgency, the approach required coalition forces to work closely with Afghan leaders to stabilize entire regions, rather than simply attacking insurgent cells.

But there was a major drawback, a new unpublished Army history of the war concludes. Because the Pentagon insisted on maintaining a “small footprint” in Afghanistan and because Iraq was drawing away resources, General Barno commanded fewer than 20,000 troops.

As a result, battalions with 800 soldiers were trying to secure provinces the size of Vermont. “Coalition forces remained thinly spread across Afghanistan,” the historians write. “Much of the country remained vulnerable to enemy forces increasingly willing to reassert their power.”

That early and undermanned effort to use counterinsurgency is one of several examples of how American forces, hamstrung by inadequate resources, missed opportunities to stabilize Afghanistan during the early years of the war, according to the history, “A Different Kind of War.”