The very conservative Cato Institute has a very well reasoned article, on how/why ISIS came about.
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To their credit, Brands and Feaver do acknowledge, in the conclusion, that “the most fateful choice was also the oldest one: the decision to invade Iraq in 2003, followed by mismanagement of the occupation” (p. 41) but they then temper that note by arguing that “it is not correct to claim that the invasion of Iraq set in motion forces that led ineluctably to the problems that the United States has faced since mid-2014.”
In a strict sense, of course, this is true. Other things could, in theory, have happened to blunt the rise of ISIS. But only a decision not to invade Iraq in 2003 would clearly and unequivocally have averted the rise of ISIS. The reason is simple: the single clearest cause of the rise of ISIS was the invasion of Iraq. As President Obama
explained in 2015, “ISIL is a direct outgrowth of Al-Qaeda in Iraq, that came out of our invasion, which is an example of unintended consequences, which is why we should generally aim before we shoot.” David Kilcullen, who worked on counterterrorism at the State Department in 2005-06 and was senior adviser to General David Petraeus at the height of the Iraq surge in 2007-08,
put it even more bluntly: “There would be no ISIS if we had not invaded Iraq.”
Was the Rise of ISIS Inevitable?
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In other words, ISIS would of not of happened, if George W would not have invaded Iraq and mismanaged Iraq following the removal of Saddam.
ISIS, gained strength later through Obama decisions.
I think that sums up reality pretty intelligently and without political bias.
Two presidents and one mistake.