Judicial Watch proved it. Under a Freedom of Information Act request, Judicial Watch was able to obtain a (heavily redacted) copy of the Defense Intelligence Agency (DIA) directive that initiated the creation of ISIS in 2012. the DIA report states,
THE WEST, GULF COUNTRIES, AND TURKEY [WHO] SUPPORT THE [SYRIAN] OPPOSITION . . . [SUPPORT] ESTABLISHING A DECLARED OR UNDECLARED SALAFIST PRINCIPALITY IN EASTERN SYRIA . . . IN ORDER TO ISOLATE THE SYRIAN REGIME . . .. https://www.lewrockwell.com/2016/08/roger-stone/isis-made-usa/