The Pentagon has made clear that the U.S. will leave Afghanistan when the rag-tag Afghan security forces have been beefed up to the point where they can keep the peace without help. "Significantly expanding [Afghanistan's national security forces] is, in fact, our exit strategy," Defense Secretary Robert Gates told U.S. troops in Kandahar last week. But that's a strategy that could leave U.S. forces in Afghanistan for quite some time to come - for one thing, the economy of impoverished Afghanistan is unlikely, for the foreseeable future, to sustain an army big enough to guarantee the country's security. And that's just one of several thorny issues likely to make success in Afghanistan harder to achieve than Iraq - unless the U.S. scales back its ambitious goals for the country. But such a rethink may be on the cards, U.S. military officers say, as internal U.S. reviews and President-elect Barack Obama give the seven-year war a fresh look.