The truth of this was disputed in the first debate.
As with almost everything politics-related, there's shades of gray between the two poles of "He opposed the war" and "He supported the war".
Before the war, he made what he calls a "very light" statement in favor of it to Howard Stern on 9/11/02, which reads to me, as if he hadn't given much thought to it either way.
"Yeah, I guess so. You know, I wish it was, I wish the first time it was done correctly."
Then, in January of 2003, he avoided giving an answer, instead saying "Either you attack or you don’t attack," and calling on Bush to make a decision. (See the first article for the full transcript).
After the war was declared, he made increasingly negative statements about it.
So, did he support the war?
Aside from what seems to be a vague statement of agreement, Trump definitely did not support the war, but there's no evidence that he was opposed to it beforehand, either.
- Did Donald Trump support the War in Iraq? Not really.
- Did Donald Trump oppose the War in Iraq? Yes.
- Did Donald Trump oppose the War in Iraq before it began? No.
- Does the record show that Donald Trump opposed the War in Iraq before it began? Definitely not.
If he maintained he opposed it in general (i.e. as it was happening), no one would question him. If he simply maintained that he opposed the war before it began, there would be reason to question it, but no one can really get inside his head to know.
Unfortunately for him, he's making the fourth claim, which PolitiFact rates as purely False: The record does not show it.