Congress can force withdrawal from Iraq, according to Glenn Greenwald.
Here is an excerpt:
"When Bill Clinton was President, most of the country's leading Republicans did not seem to have any problem at all with Congressional "interference" in the President's decisions to deploy troops (really to maintain troop deployments, since President Bush 41 first deployed in Somalia). There wasn't any talk back then (at least from them) about the burden of "535 Commanders-in-Chief" or "Congressional incursions" into the President's constitutional warmaking authority. They debated restrictions that ought to be legislatively imposed on President Clinton's military deployments and then imposed them."