The war in Afghanistan — America’s longest conflict — will grind on for at least another four years as NATO allies are prepared to commit $5 billion through 2020 to train, equip and pay Afghan security forces, according to a senior NATO diplomat. Last week, President Obama granted U.S. troops in Afghanistan expanded authority to attack Taliban insurgents. The new rules allow U.S. forces to advise regular Afghan combat units and to call in airstrikes. There are about 9,800 U.S. troops in Afghanistan, and Obama has pledged to reduce their number to 5,500 by year’s end.