Kerry authorized the president going to war, and believes that a president should have that power. There are a few things he would have done differently, though.
One of them is the money needed for this war. Kerry voted against American taxpayers having to pay $87 billion. In stead, he'd have brought in the allies and share the burden. Bush talks a lot about those allies, but let's face it: the 21,000 non-American troops that have been in Iraq are a joke (not the troops themselves, but that number). Only a little over 30 countries have provided troops. That's not "the alliance", that's a small part of what the alliance should be and has been before Bush.
Also, Kerry would add 40,000 active duty troops, and end the backdoor draft of National Guard and reservists, double the special forces (the ones that should've been send to Iraq instead of "newbies")