According to the latest CBS poll on Afghanistan (PDF, page 3), whether or not you a voter approves of President Obama's handling of Afghanistan has as much to do with partisan self-identification as with President Obama's actual policies in the country.
By a 2-1 margin, self-identified approve of President Obama's handling of Afghanistan, even though twice as many Democrats favor decreasing troop levels there as favor increasing troop levels.
President Obama has a net approval rating of negative 38% among self-identified Republicans in Afghanistan, However, three and a half times as many Republicans think troop levels in Afghanistan should be increased rather than decreased.
This partisan discrepancy can probably be explained by Afghanistan being low on the list of national priorities right now. In the same CBS poll, only 3% of Americans cited Afghanistan as the top national priority. As such, many Americans are relying on their partisan preference to determine their approval or disapproval of President Obama on Afghanistan, rather than their personal views on how many troops we should have in that country. When an issue is not high on your list of national priorities, it is a safe bet that a given voter less likely even be aware of a difference with his or her party leadership, and more likely to let a difference s/he does notice slide.
There are possible readings of these numbers which are a lot less generous to the electorate, but I won't go there in this article. As tempting as a "stupid electorate" conclusion is from numbers like these, in my experience it is best to withhold such conclusions until there are no other possibilities.