ISIS exists because Saddam was toppled, replaced by, a mostly shia government and the old iraqi military was disbanded. That created a power vacuum and left a lot of military trained guys unemployed, who felt betrayed. They basically said "fuck this world order, we will make our own, with ~~blackjack~~ jihad and ~~hookers~~ sharia law".

Over time, they became more and more religious radicals, split off Al Qaeda and now they are fighting everyone everywhere. ISIS-K(the one in Afghanistan) seems to be the main one organizing terrorist attacks now. They are fighting the Taliban, Iran and Pakistan too. Most of ISIS' victims are muslims, sometimes shia but often sunni(but not the right sunni).

They are a very attractive proposition for poor and desperate people around the world, who feel they got screwed. They are basically resentment manifest.

Now you might say "thats because of western imperialism" and you wouldnt be wrong. But do you know what was the most common ethnicity for foreign ISIS fighters? Russian. And no, it wasnt random russians fighting against american imperialism, it was muslim russians who simply could not fight in Russia's Dagestan and Chechnya. Russia bombed the shit out of ISIS too, partially because of this reason.

While understanding what lead us to our current situation is important, ultimately we must face the present. Whether a military solution is the best way, is debatable. But it is easier to convince your parliament to spend x amounts of money on your own military, rather than use that funds on developing other countries.