Well of course like much of history the situation here is complex and has a long and textured point of origin, I will give you a summary as best I can:
The middle east is a region which was, like all of the world outside of Europe, colonised by European forces. The borders of the Middle East are essentially makeshift, and Iraq was a state founded by the Sykes-Picot agreement by France and Britian. This artificial drawing of boarders means that in many middle eastern regions there is a mixture of different race, religions and creeds. Iraq sits at the forefront of such nations, with a minority Sunni, Majority Shi'ite Muslim demographic. Not to mention Kurds, Yazidis etc.
This scenario is something which has caused civil wars all over the post-colonial world. What makes the Middle East more prominent is the level of interference it has received in its post-colonial era. It has been subject to constant disruption, puppetry and occupation by Western Nations and many postulate the reasons as to why. Some would say that Oil makes the region geo-politically relevant, others that places like America have a perceived duty to Israel and will therefore destablise the region as much as possible.
Over the years there have been many oppressive, regressive Islamic regimes that have sprung up in the region and which have been supported by the Americans (at least in their preliminary stages) such as the Shah of Iran. These were regimes which stood against values of individual rights, and were puritanical and despotic in their views of leadership. Yet they were allowed to thrive and were supported for reasons we can only speculate on, though one would assume it was for some sort of financial or political gain.
Anyway lets skip to Saddam Hussein. Saddam was a Sunni and by virtue of the fact that Iraq was majority Sh'ite he ruled with an iron fist to keep power. An inevitability in an artificial state. Unfortunately the will to power in a state which is not self-determined and is made up of many sects is through use of oppression. Under Saddam, and with oil, the nation of Iraq thrived. Saddam now harbored expansionist aims. Typically the Americans supported him (armed him) as he went into invade Iran (no longer friends with the US). This was in spite of the fact that Saddam was clearly no saint. Eventually this American foreign policy changed when Saddam entered Kuwait with his expansionist aims and the first Gulf War kicked off.
Obviously the Americans left Saddam in charge, much against the wishes I am sure of many senior Americans due to international opinions on the conflict. So what did they do instead? They imposed economic sanctions so severe on Iraq that the country went into severe economic decline which resulted in the preventable deaths of over 1 million innocent Iraqis. This is relevant because this pro-Israeli imperialist aggression from the USA, allowed a form of hatred to rise so much in the region it has essentially spawned Al-Qaeda and Isis and the like.
So now we have a scenario in the late 90's early 2000's were America are squeezing the life out of some of the Middle East. Not only that but they decide to get into bed with some fairly backward (at least by American ideologies) states like Saudi Arabia. So essentially you have the more secular states like Syria and Iraq on the Americans list of hated countries, and complicit Islamic states like Saudi Arabia as their best friends. In my opinion this is because America wants nothing more than 'yes' men and instability in the region. Of course in these countries, which are traditionalist Muslim, America is still not liked by the masses even if they are in bed with the ruling elite, Al-Qaeda is essentially a Saudi entity, and these are individuals who due to the actions of America/Israel in their backyard have formed Jihadist tendencies and strike out with viscous acts of violence (9/11). Of course America can't invade Saudi Arabia, where the violence came from. So they use this as an opportunity to go after Saddam, who had absolutely nothing to do with this Jihadists movement. Sure enough they occupy Iraq and depose him. Then they continue to occupy, meanwhile giving more proverbial ammunition to Jihadis by being an occupier in their region again. These states like Saudi Arabia and Qatar have rich individuals, who are anti-Israel and anti-America and they start pumping money into these Islamo-Fascist institutions like Al-Qaeda.
Eventually the Americans have to leave, and at this same time populist uprisings start happening in the Arab world. Eventually this spreads to Syria and eventually an anti-regime civil war is hijacked by these well funded, well armed Jihadis. Who eventually become the dominant player in Syria, and then move into Iraq which is now not occupied by the USA. So out of these hardcore Islamic groups spawns ISIS. The most war mongering and vengeful of them all. Who are Sunni (salafist/wahabist) groups hellbent on destroying anyone who doesn't believe in their strain of Islam. They are the spawn of years of horrible Western foreign policy/occupation in the region. But I am not excusing their behavior, they are cnuts.
@Kaos @Raoul I'm sure would explain some areas a little better, as would I but there is a lot to type.
at least he doesn't really try to hide it.