(CNN) -- What's happening now in Iraq is dramatic, significant, quite possibly historic. But, to some, it is not surprising.
Militants believed to be from the Islamic State in Iraq and Syria, also known as ISIS, overmatched government forces and now control a vast swath of its territory. Hundreds of thousands have fled, becoming refugees overnight. Sectarian violence plagues some areas not under ISIS control.
And amid all this, some believe the Baghdad-based central government won't be able to do much about it.
Some of these developments, like the fall of Mosul, have been swift and sudden. Others, such as the tensions between Shiite and Sunni Muslims, date back for decades, if not centuries.
But all this trouble didn't come out of nowhere. For years, experts have predicted that various factors -- some rooted in history, some of them related to recent big decisions, some functions of what's happening in the region -- could foster instability and violence in Iraq.
While not as big as what it had been prior to the 1991 Gulf War, Iraq's military under Saddam Hussein boasted an estimated 430,000 soldiers and another 400,000 personnel in paramilitary units and security services when U.S.-led troops invaded in spring 2003.