From The White House, July 13:
Ordering the Selected Reserve and Certain Members of the Individual Ready Reserve of the Armed Forces to Active Duty
By the authority vested in me as President by the Constitution and the laws of the United States of America, including sections 121 and 12304 of title 10, United States Code, I hereby determine that it is necessary to augment the active Armed Forces of the United States for the effective conduct of Operation Atlantic Resolve in and around the United States European Command’s area of responsibility....
....MUCH MORE
Here's the Atlantic Council's description of Operation Atlantic Resolve
