Not a rag tag militia anymore. These are people trained by intel agencies of the world to fight their wars.
ISIS evolved in Syria and Iraq from rogue groups within FSA. Now remember who trained FSA? ISIS was then joined by Al-Nusra. Think of Al-Aqaeda and multiply by 10 to know brutality of al-Nusra.
these people are trained military grade. They have finances from upstarts of Arabia - Qatar, Kuwait, UAE and Bahrain. Dont discount US or CIA. They all have their hands dirty. At some point Saudis too were funding them but when they saw them becoming stronger in their own country, they pulled out.
Watch this documentary. Its an sanitized version of American naivety. At every point in the documentary it tries to wash American hands off this brutality. But US does have a major hand in this, It created the ground work where such a poisonous harvest was sowed.
Do watch it
The Rise of ISIS | FRONTLINE | PBS