If Kerry needs criticisms to hurl at Bush, it is unacceptable that Bush didn't muster the international coalition of the first Gulf War, which he opposed (before he was for it). If there are no WMDs in Iraq, Bush misled us into war, even though Kerry himself said the same thing about WMDs, misleading the public into supporting a war that he would support only for as long as he didn't oppose it.
If pressed to say whether he would have voted to authorize the war despite not finding WMDs, he supports voting the same way, authorizing a war that he eventually opposed, even though it deserved his authorization vote and still does today, despite his opposition to it. If criticizing Bush's postwar management, he supports more troops in Iraq. If criticizing Bush's postwar management, he supports pulling troops out of Iraq in six months.
If desperate to gain post-August traction against Bush, he thinks Iraq is "the wrong war, in the wrong place, at the wrong time." Although he would vote again to authorize the wrong war, so long as he wouldn't be committed to voting to fund it, at least not committed to voting to fund it in a circumstance where he couldn't immediately also vote against funding it to demonstrate how he opposed the wrong war he supported and would authorize again.
See? It's simple.