According to http://www.telegraph.co.uk/news/worldnews/middleeast/syria/1... there are already trainees betrayed the U.S. and are probably on ISIS side. On the front of Afg and Iraq wars, there have been cases in which U.S. trained Afg and Iraqi soldiers stood by the insurgents.

And Saddam Hussein was funded by U.S. to rebel against Iran in the 60s, 70s.