Who decides troop levels?

Military commanders should be deciding troop levels, Bush said. "I know what I'm doing when it comes to winning this war, and I'm not going to be sending mixed signals," he said.
Compare Bush's statement with this post by Kevin Drum.
So the Army Chief of Staff, Eric Shinseki, thought we'd need "several hundred thousand troops," and the CENTCOM commander on the ground estimated 250,000 troops. But we went in with about 150,000. And as Spencer points out, Rumsfeld and Paul Wolfowitz have insisted ever since that this number was exactly what the military commanders advised — no more, no less. I know we're not allowed to say that anybody in the Bush administration lies, but Rumsfeld and Wolfowitz sure seem to have a creative way with numbers, don't they?
Now if I was running Kerry's campaign I'd make it a point to ask the President about this apparent contradiction.