Courtesy of Foreign Affairs:
Benjamin Friedman has a good op-ed in the Christian Science Monitor. He criticizes the field of presidential candidates for seeking to expand the western troop presence in Afghanistan, either through NATO or the American military.
His point in a nutshell- foreign military power is no substitute for a legitimate federal government. The US doesn't need a large occupation force to conduct anti-terrorist operations, and military forces are terribly ineffective at nation building. Hence sending in more troops is a waste of lives and money.
Friedman does not dwell on the political calculus that encourages both parties to support escalation, but it's not hard to see. First there is a perception that while Iraq is 'lost' Afghanistan can still be 'won', and having more troops there increases the chance of winning. Afghanistan is salve and distraction from our losses in Iraq. Escalation also serves as political cover; candidates who would withdraw troops from Iraq can still claim the mantle of militarism by sending them elsewhere. There is a third reason for sending troops to Afghanistan, one that somewhat undermines Friedman's argument.
I'd argue that eastern Afghanistan and Pakistan are more important to America's national security then Iraq. First, Pakistan is an unstable country with nuclear weapons. Anything is possible there, and a significant area troop presence creates options. Second, the worst people are there. Osama Bin Laden and his followers, the people who actually did attack the U.S., are there. More troops permit more effective cordon and search operations. In sum, Afghanistan and Pakistan possess real WMD, and real people who have demonstrated the will and ability to launch attacks on U.S. soil. Iraq (and Iran) possess neither.
That's a relatively small problem though, because that only works if you think military forces in Afghanistan won't be used for nation building, and they almost certainly will be. It's worth thinking about what that is likely to accomplish.