Patently false.
Someone here posted a video of Trump in January 2003, that's before the invasion of Iraq, saying he was opposed.
Even in the video you all like to point to, he wasn't exactly for the war with any conviction.  "I suppose," he said.  Unlike you-know-who, who took to the floor of the senate and proudly cast her vote for the war, and "with conviction."
http://www.foxnews.com/politics/2016/09/27/2003-clip-backs-up-trump-on-iraq-war-opposition.html
2003 clip backs up Trump on Iraq War opposition
(video at the link)
But Cavuto himself picked up the thread post-debate on Fox Business Network, unearthing the clip Trump referenced, from January 28, 2003 – Nearly two months before the Iraq War began on March 20. In the video, Cavuto asks Trump how much time President Bush should spend on the economy vs. on Iraq.
“Well, I’m starting to think that people are much more focused now on the economy,” Trump said. “They’re getting a little bit tired of hearing ‘We’re going in, we’re not going in.’ Whatever happened to the days of Douglas MacArthur? Either do it or don’t do it.”
Trump continued: “Perhaps he shouldn’t be doing it yet. And perhaps we should be waiting for the United Nations.”