Gen. David H. Petraeus, the head of the United States Central Command, said Sunday that the battle being fought in the Taliban stronghold of Marja was the “initial salvo” in a military campaign that could last 12 to 18 months. In an interview on NBC’s news program “Meet the Press,” the general, who oversees the wars in Afghanistan and Iraq, sought to put the Marja battle “into context” by explaining its part in the overall effort by American, Afghan and other forces to defeat the Taliban. He said that international forces had spent recent months mapping strategy, gathering resources and preparing the leadership of a “comprehensive civil-military campaign.”