sentences:  The number of 7th Air Division bases used for Reflex reached a peak of nine in early 1959.The development of Intermediate Range Ballistic Missiles by the Soviet Union made forward bases for SAC medium range bombers increasingly vulnerable.After 1958, when the 100th Bombardment Wing departed RAF Brize Norton, SAC bombardment wings no longer rotated as entire units, although six bombers continued on alert at each of the division's bomber bases.