Bush succeeded with the invasion but failed with the occupation. He won the war but lost the peace. There was a window of opportunity in which the US could have won over the hearts and minds of the Iraqi people. That window is now gone, never to return. The Iraqis now hate the US, whereas they were willing to give it a chance when US troops swept in a year ago. This means that the US can no longer bring democracy to Iraq, and it is now obvious that the Bush administration has given up on that idea. The only way the US can now maintain order is through the use of naked force.
The reason Kerry must be clear about this is that he will inherit Iraq from Bush. Iraq is turning into Bush's Vietnam. If Kerry does not make clear right now that the Iraq war is unwinnable for the US, because unpopular occupations always fail, Iraq will be Kerry's Vietnam, too. Kerry and other Democrats must make clear to the American people that Bush has already lost Iraq because of the incompetency of his occupation, so that Kerry will be able to leave Iraq gracefully, his having to leave not being seen as his own fault.
The way Kerry is trying to play this right now is to say that the answer is more foreign troops. But it is too late for that. The resistance in Iraq is now too severe for other countries to be willing to send troops, to die for America's mistake. It is important for Kerry to realize this now: otherwise Iraq will weigh his presidency down so severely that he will be able to accomplish nothing.