Images of the World Trade Center towers collapsing in New York were still fresh in the minds of the first American troops arriving in Afghanistan, as the U.S. launched an invasion targeting the Afghanistan-based al-Qaida leaders who plotted the attacks of Sept. 11, 2001. More than 800,000 U.S. troops have served in the Central Asian country since then, in a war that quickly expanded to confronting Afghanistan's Taliban and to nation-building. On Monday, the top U.S. commander in Afghanistan, Gen. Scott Miller, relinquished his command in Kabul, underscoring the winding down of America's longes...