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Oil funds predominantly along with theft and ransoms according to the EU near ISIS's peak [1]. This was while the US was still directly fighting ISIS in Afghanistan and Iraq.
1: https://www.europarl.europa.eu/RegData/etudes/IDAN/2017/6038...
You need to be a bit more specific with exactly what you mean by “religious” extremism, you seem to be conflating things that are in many ways opposite