The US and Saudi Arabia funded ISIS.

International arms watchdog finds that most ISIS weaponry was purchased by the United States and Saudi Arabia before being shipped to opposition forces in Syria and Iraq.

A damning report by the EU-funded Conflict Armament Research has found that the majority of weapons that ended-up in the hands of Daesh/ISIS were purchased from Eastern European stockpiles by the United States or Saudi Arabia and transferred to opposition militias in Syria. In many cases there is clear evidence to suggest that the arms were transferred into ISIS possession in less than a month, as US-backed groups amalgamated into ISIS or the weapons were sold on a thriving black market.

A significant proportion of the weapons examined were unauthorised for retransfer, meaning that they had End User Clauses for US and Saudi military or police use that were quickly violated, not even being shipped to their country of authorised usage in many cases.