As with many things, it depends where you stop looking for root causes and responsibility. Typically we stop looking when we get to somebody we don't like such as an outsider, and find confirmation in our choice when a continued search starts turning up people we like.
Take Zarqawi. He was a problem agitating and working against Jordan before he fled for Iraq after 2003. Nobody likes looking back there because Jordan's not just all that bad and because Zarqawi is Palestinian and Salafist (and if not Palestinian, then a member of the Bani Hassan tribe, a conservative tribe that extends into the West Bank ... So he could be both). The Iraq War gave an opening for him to become more important. But it was not responsible for most of the important players in his organization. For that you need to trace back to Salafist activists in Jordan and Syria (who went to Iraq, and not just Iraq, and who otherwise were held in check) and to Saddam's explicit support for his power base in western/central Iraq, where Salafists used the money and influence he offered them to increase tribalism and conservative thinking. We like to cite Iraqi stats and policies from the early '80s, not from the '90s, when it come to this kind of thing.
Notice that al-Baghdadi wasn't smart enough to study what he wanted (apocryphal sources say) so he had to resort to the lesser course of Islamic studies. When and where was that? Under Saddam's "secular" regime.
Same for Syria overall. The Iraq War and how it played out let a lot of Salafist forces in Syria to get experience fighting and linking up with other organizations across the belt that stretches from Morocco through to Tajikistan. The forces were there, just untrained and without any great cause that would increase recruitment. Syria--Assad, that is--found Iraq a handy escape valve for these folk. Once the Iraq War was over, governments used rather nasty means to keep them in check--torture, executions, imprisonment, etc. We objected to that and decided to encourage the "revolution" and Syrian version of the "Arab Spring" (Note that we use bombs, bullets, and detention camps to keep these forces in check, along with imprisonment, and say how much better we are.)
So in part Syria didn't mind their Salafists going abroad for training and expertise; they just expected that when they got back--if they got back--they'd be managed by the usual means. Instead they were partly defended. (And they're often part of the Turkish-backed Islamist forces we see attacking Kurds, as well as some of the Xian groups that were genocided a century ago by the Turks.)
No Syrian "Arab Spring", or one quickly crushed? No ISIS. (Sorry, that stops too soon.)
No Iraq? It's likely ISIS wouldn't exist. (Ah. Blame Bush II.)
No Syrian passive support for home-grown Islamist forces heading to Iraq and elsewhere? No ISIS. (We liked viewing the anti-US forces as "freedom fighters" and "home grown." And this was when a House delegation visited Assad and spoke well of him because Bush II was talking smack.)
No Saddam Hussein? No ISIS. (Tough one. Saddam killed Kurds; Saddam was a victim. Can you be a guilty victim?)
Even weirder is the Turkish support for many Islamist groups, and the fact that a lot of ISIS weapons were Russian--perhaps not supplied by Putin, but routed through the Ukraine, almost certainly the Donbas. That doesn't mean Putin's knowledge, because a lot of materiel was chucked into the Donbas and Russia was a bit peeved over some of the "leaders" in 2014 and 2015. Yanukovich was also supported less for all that he did and more for certain things that he did.
Etc. Etc. Etc.
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