KABUL, Afghanistan — American and NATO troops closed their operational command in Afghanistan on Monday, lowering flags in a ceremony to mark the formal end of their combat mission in a country still mired in war 13 years after the U.S.-led invasion toppled the Taliban regime for harboring those responsible for 9/11. The closing of the command, which oversaw the day-to-day operations of coalition combat forces, is one of the final steps in a transition to a support and training role that begins Jan. 1. But with President Barack Obama's recent move authorizing U.S. forces in Afghanistan to carry out military operations against Taliban and al-Qaida targets, America's longest war will in fact continue for at least another two years.