During Saddam's era sunni and shia lived well together with no problems. But America attacked Iraq and supported shia extremists groups like hezbollah that turned the war into a sectarian conflict. Sunnis felt betrayed by shia who supported the American invasion, so Al Qaeda took advantage of the situation to enter Iraq and create its own state.
The truth is that the shia-controlled parts of Iraq really resemble a shia caliphate, but this shia caliphate is well tolerated because the government is a puppet of USA. This makes clear that Iran and USA and not enemies. Shiite groups like Hezbollah easily demonize USA but they're the first to ally with them when it comes to gain power to rule over sunnis. Why don't they attack Israel? should the sunni majority support Hezbollah and give away their rights just because they say that they're against Israel, even if they don't take any action to fight it? I don't think so.
Despite islamophobics' articles and theories, the truth is that for ISIS we have to blame no one but America. They created a sectarian war. Iraqis just wanted to live in peace.
Sunni iraqis don't hate christians, but they have no choice: if they travel to the shia-controlled part of Iraq they risk to get killed by hezbollah, so they prefer to stay in ISIS controlled zones, even if they're secular and they don't like to live in a caliphate, at all.
