The Dem candidates' weird answer on Iraq withdrawal

When asked at the recent Democratic presidential debate about whether we'd be withdrawing all combat troops from Iraq by the end of their first term (i.e., in 5 years), neither Obama, Hillary, or Edwards were willing to say "yes." What disturbs me is that they could have easily given a qualified yes. Qualified because it is entirely possible that we'll need to leave some troops in Iraq, say a few thousand, for various duties. But that's a far cry from Bush's current bottom line of some 140,000 or so troops that we MAY get the number down to some time next year. So why couldn't any of the top three say "yes, though we may need to leave a small number in place for a variety of duties that don't include major combat operations as they do now"? And especially Edwards and Obama. If they wanted to shake things up,