Why can't Congress order the army to leave Iraq?

Can one of the many Constitutional law geniuses on this site answer this question for me. The question popped up on another blog and I don't want to make a fool... or bigger fool... out of myself on this issue.

I get that the Constitution makes the President the Commander and Chief of the armed forces.  However, Article 1, section 8 sez:

The Congress shall have Power...

To make Rules for the Government and Regulation of the land and naval Forces;

.  

So what I don't get is why can't Congress simply "regulate" that the Army (etc.) to leave Iraq by a certain date?

a little more after the jump.

Yeah, I know Bush would automatically veto something like that. But there are bills Bush must sign, so it wouldn't be the first time that Congress  mashed some stuff the President didn't like into a Bill he had to sign.

Which leads me back to the title...why can't Congress (with Bush's signature and ignoring any more retarded signing statements or filibustering), provide the Army with a legally enforceable timetable to leave Iraq?  What am I missing?

[note: I'm not recommending that we run our military this way, I'm just asking if it would be Constitutional?]

Anyway, thanks for reading.