Although this has been known for some time,
this WashPo article does a good job of explaining how what we now call ISIS consists largely of Saddam Hussein's former military leaders and Baathists - particularly at the leadership level.
A better understanding of the former Iraqi army's connections with ISIS is especially important because so often this latest military conflict is described as a "new" fight against "foreign terrorists" or even a joint-Iraqi effort to "liberate" Sunni cities and territory within Iraq. The truth is much more complicated, and the U.S. military's strikes against ISIS are very much a continuation of the original Iraq occupation and the internal Iraqi conflicts unleashed by our invasion.