Neither the pro nor anti factions really anticipated how things would shake down in Iraq, so the failure of a billionaire, political dilettante to predict the outcome is hardly surprising. Trump is giving himself more credit for prescience than he deserves, but he clearly had reservations before the start of the war, which turned to outright condemnation once the dust of the occupation began to settle.
In any case, the question of how, and to what degree, the invasion 'destabilized' the Middle East is debatable. The proximate cause of the present dire situation was the Arab Spring, which the political left in Europe and America welcomed wholeheartedly. What role did the Iraq Invasion play in the genesis of the Arab Spring, or was it an inevitable eruption of popular discontent which would have taken place if no American soldier had ever set foot in Iraq? The situation, at least in Syria and Iraq, would have been wholly different if the Obama administration hadn't been so anxious to cut and run, and hadn't meekly acquiesced in the ejection of all American troops from the country.
