Iraq received warnings about the “growing threat” from Sunni militants in Iraq as early as the beginning of 2014. US intelligence agencies say:
During the past year, the intelligence community has provided strategic warning of Iraq’s deteriorating security situation. We routinely highlighted (ISIS’) growing threat in Iraq, the increasing difficulties Iraq’s security forced faced in combating (ISIS), and the political strains that were contributing to Iraq’s declining stability…We knew exactly what strategy they were going to use, we knew the military planners.
Aside from the warnings from US intelligence agencies, both The Telegraph and Daily Beast are claiming Kurdish sources did warn American and British officials that ISIS was gaining strength and was ready to advance, but it “fell on deaf ears.”