Spencer Ackerman has a good introduction to the divide among the progressive Left and centrist Democrats over the US-led war in Afghanistan. The far left opposed the initial bombing and semi-invasion of Afghanistan, largely on humanitarian grounds (see sentiments expressed in Oct. 2001 here). With the experience of the Iraq war, more people from the center-left are skeptical that greater foreign intervention will improve conditions in Afghanistan. As the insurgency and counter-insurgency there intensify, their opposition increases.
Their concerns over US policy are reasonable. Many of the pragmatic reasons against invading and occupying Iraq apply to Afghanistan. First, like Iraq, Afghanistan is ethnically divided, with one ethnic group, the Pashtun, having strong external support, much like the Sunnis in Iraq had from predominantly Sunni Arab states. In this case, Pakistan, with a sizable Pashtun population itself, backs them. Ethnic division is not inherently conflict-prone, but when those divisions reflect historic political and economic inequalities and when the country is poor, they are sources of conflict.
Second, Iraq’s economy depended on a single commodity export; so does Afghanistan’s, but rather than oil, Afghanistan exports opium. Just as South American farmers find coca more profitable than foodstuffs, Afghan farmers find poppies to be the most economical crop. Given the low price of commodities in the current global downturn and continued developed-world farm subsidies, this is unlikely to change soon.
Third, because Afghanistan has a larger population than Iraq, the recommended soldier-to-civilian ratio will require more soldiers in Afghanistan than at the height of the surge in Iraq. The only hope is that Afghan security forces can increase sufficiently that fewer foreign troops are needed than in Iraq. But that does not seem likely at this point, given the levels of corruption within the Afghan government.
I still have not addressed the ethical arguments against a counter-insurgency that depends heavily on bombing to support ground forces. Many on the left find these compelling reasons to end the war in Afghanistan.
Of course, centrist Democrats, which includes the Obama administration, believe that Afghanistan is the true central front in the war on terror. One their objections to the Iraq war was that it distracted attention from eradicating al Qaeda and stabilizing Afghanistan. These differences could be ignored when ending the Iraq war was the priority. But now that Obama is fulfilling campaign promises to return US military attention to Afghanistan, leftist opposition is increasing.