The Islamic State militant group (ISIS) got its hands on vast supplies of weapons by taking advantage of U.S. weapons transfers that may have violated international agreements between Washington and its allies, according to a new report.

As much as 90 percent of ISIS’s arms and ammunition were found to have originated in Russia, China and Eastern European states. The jihadis were able to obtain much of this arsenal as a result of former President Barack Obama’s support for rebels in Syria, U.K.-based Conflict Armament Research reported after analyzing 40,000 items recovered by its investigators along ISIS front lines between July 2014 and November 2017. By purchasing “large numbers” of European arms and ammunition and then diverting them to nonstate actors in Syria without notifying the sellers, the U.S. reportedly “violated the terms of sale and export agreed between weapon exporters...and recipients."

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“The United States and Saudi Arabia supplied most of this materiel without authorization, apparently to Syrian opposition forces. This diverted materiel, recovered from IS forces, comprises exclusively Warsaw Pact–caliber weapons and ammunition, purchased by the United States and Saudi Arabia from European Union (EU) member states in Eastern Europe,” the report found, using an alternative acronym for ISIS.