The only evidence they have that Trump supported the war is that 2002 interview with Howard Stern where he kinda/sorta said he approved. Most of his comments after that have been either neutral or outright opposition to the war.
He should just say “Really? You want to take an off the cuff discussion with HOWARD STERN as my approval of something as serious as a war? That is how low you have sunk?”
And that was when it was just an idea being floated -- Congress hadn't approved it yet, provided funding or sent troops.
It doesn’t matter what he thought. He wasn’t holding any public office and had no influence on what happened. He was just a private citizen expressing an opinion.