I believe Mr. Trump feels that Bush elevated the threat of WMD to a level that it shouldn't have been (i.e. nukes). There is no doubt that Hussein had chemical weapons at one time and he probably possessed bioweps as well. Still, there were some in the intelligence community and some inspectors (Scott Ritter for one) who were arguing that there were no WMDs in Iraq.
My disatisfaction with Bush isn't over the invasion of Iraq, but over the execution of the war. It is evident (IMO anyway) that he wanted to keep his tax cuts in tact by allowing Rumsfeld, and directing Casey, military et al to fight a limited war (war on the cheap) instead of employing the proven military tenent of overwhelming force. Limited war resulted in the foreign Al Qeida terror groups being allowed to pour in over the unsecured border and sparked the insurgency after the fall of Baghdad. Together with the looting, crime and widespread anarchy it will serve as Bush's lasting legacy of failure as Commander in Chief.
I beg to differ some. I saw Rumsfeld stumping for a 'small footprint'. When folks like McCain were saying we need a surge, I wanted the RINOs to sit down and shut up. But they were right and Rumy and I were wrong. Bush replaced Rumsfeld with Gates and Petraeus and a job well done to them all and most importantly a job well done to our Troops.
Every war has its mistakes, but let it be known that our military went faster and further behind enemy lines and removed the leaders quicker then any military in the history of the world. With the least amount of people dead too. The execution of 'the War' was excellent. We have a problem with insurgents. Germany had bombs going off five years after the end of WWII. We are doing OK in Iraq, we have to keep at it. The basic execution was sound. We had to adapt, improvise, and overcome a few situation, but that is in any war. Now we have to finish the job so we never have to go back.