Was kerry forced to move left because of Dean? Like how Romney had to move right because of the party?
Kerry won the primary on perceived competence and electoral strength, not by winning the allegiance of any particular set of issue voters.
And after the Bush Administration's dismal handling of the war in 2003 and 2004, I don't see how any challenger couldn't have made hay of that. If Gore had been elected, and had prosecuted the war with such incompetence, the Republicans would have taken Kerry's position. Unfortunately for Kerry, the public didn't turn against Bush quickly enough.