The 556th Strategic Missile Squadron (551 SMS) is an inactive United States Air Force unit. It was last assigned to the 820th Strategic Aerospace Division, stationed at Plattsburgh AFB, New York.The 556 SMS was equipped with the SM-65F Atlas Intercontinental ballistic missile (ICBM), with a mission of nuclear deterrence. It was the only ICBM squadron east of the Mississippi River. The squadron was inactivated as part of the phaseout of the Atlas ICBM on 25 June 1965.