This led to a series of systems starting with Nike Zeus, then Nike-X, Sentinel and finally the Safeguard Program. These systems used short and medium-range missiles equipped with nuclear warheads to attack incoming enemy ICBM warheads. The constantly changing concepts reflect their creation during a period of rapid changes in the opposing force as the Soviet ICBM fleet was expanded. The interceptor missiles had a limited range, less than , so interceptor bases had to be spread across the United States. Since the Soviet warheads could be aimed at any target, adding a single ICBM, which were becoming increasingly inexpensive in the 1960s, would (theoretically) require another interceptor at every base to counter it.