Jonathan Chait: "[Jonathan] Schell insisted [in 1990] that we could force Iraq to leave Kuwait with sanctions alone, rather than by using military force. But the years that followed that war made it clear just how impotent that tool was. Saddam Hussein endured more than a decade of sanctions rather than give up a weapons of mass destruction program that turned out to be nonexistent. If sanctions weren't enough to make him surrender his imaginary weapons, I think we can safely say they wouldn't have been enough to make him surrender a prized, oil-rich conquest."
So Chait says that sanctions obviously were a failure, because they weren't enough to make Hussein surrender the weapons he didn't have.
Not only that, they also failed to make him draw square circles and build perpetual motion machines!
(Via the below-mentioned Arthur Silber.)