Soon after 4,000 U.S. marines flooded into Afghanistan's Helmand River Valley on July 2, Mullah Abdul Ghani Baradar called top Taliban regional commanders together for an urgent briefing. The meeting took place in southwestern Pakistan—not far from the Afghan border but safely out of the Americans' reach. Baradar told the commanders he wanted just one thing: to keep the Taliban's losses to a minimum while maximizing the cost to the enemy. Don't try to hold territory against the Americans' superior firepower by fighting them head-on, he ordered. Rely on guerrilla tactics whenever possible. Plant "flowers"—improvised explosive devices—on trails and dirt roads. Concentrate on small-unit ambushes, with automatic weapons and rocket-propelled grenades. He gave his listeners a special...