Bush decided to invade Iraq.
The decision was taken without taking the consequences of this action into account. The US thinks it can go in, beat someone up, and everything will be fine and happy.
The US went into Iraq thinking the people would celebrate the US as saviours. They didn't. Anyone with half a brain would have seen that this wouldn't have happened. But Bush doesn't have half a brain.
Then something even worse happened. Paul Bremer was supposed to be in charge of the military side to Iraq. Some other guy, a Muslim and a guy from the region, was supposed to be the Muslim side of things. This was all agreed upon, all Bush's advisors knew this was going to happen.
Bremer went to Bush and said he should be in sole charge. Bush agreed and gave Bremer control of Iraq. There was no discussion, no debate, etc with Bush's advisors, ie, intelligent people who had more of a clue.
So Bremer was in charge.
First thing Bremer did (more or less) was to disband the Iraqi Army and Police. Anyone connected with the Saddam regime was to not be allowed to be a part of the new regime.
So, all these men were made unemployed. They became fighters against the US, against an Iraqi Army and Police which didn't have a clue as no one had experience. They relied on the US and it's not so strong force to oppose the new found insurgents, made up of people Bremer had fired, instead of paying them to stay on his side. This cost the US so much more money that it would have cost them to keep them in the Iraqi Army and Police.
That's how Bush helped to cause ISIS from the US side. Now let's look at ISIS.
ISIS started out in 1999 as
Jamāʻat al-Tawḥīd wa-al-Jihād, founded by
Abu Musab al-Zarqawi.
Al Zarqawi was a poor Palenstinian-Jordanian. He was a violent criminal, the sort you see in most countries and the sort you try and avoid. The sort that would have ended his life in obscurity if he hadn't been given plenty of chances to make a name for himself.
He went to Afghanistan in the late 1980s to fight the Soviets. But they were leaving when he arrived. But he met a certain Osama bin Laden there.
He started what would become ISIS in 1999 because he was released from Jordanian prison. He was still a thug and all of that, nothing much more.
In 2001 the US invaded Afghanistan, al Zarqawi then went to fight the US. But he got injured and left, though probably with a taste for fighting.
In 2003 the US invaded Iraq and this is where he grew into what he'd become.
ISIS was born out of the fighting in Afghanistan and Iraq, which Bush initiated, because he wanted war.
ISIS learned its trade in these wars. It's hardly surprising they're still going. The destabilised region was a perfect breeding ground for such militant groups.
It all happened because of Bush's invasion of Iraq and the disastrous post war period f**k up by Bush's man Bremer.