The operation marked the first known instance of American service members battling ISIS fighters on the ground in Iraq under President Obama's new mission to "train and advise" local forces against the terror group.
To summarize the article real quick, in short the US got intel that ISIS was...
U.S. "accidentally arms" ISIS because too afraid to make sure weapons reached intended group:
U.S. accidentally delivered weapons to the Islamic State by airdrop, militants say