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"America does not forget, our reach is long, we are patient, justice will be done," Obama told veterans gathered at a convention of the American Legion in Charlotte, North Carolina.
Obama, who ordered airstrikes against the militant group in Iraq and may expand them into Syria, said he would do whatever is necessary to go after those who harm Americans.
"Rooting out a cancer like ISIL won't be easy and it won't be quick," he said, referring to an acronym the United States uses for Islamic State.