Union originally started by President Lyndon B. Johnson were continuing. The Anti-Ballistic Missile Treaty of 1972 limited the US and Soviet Union to two ABM sites each. Safeguard was scaled back to sites in North Dakota and Montana, abandoning initial work at a site in Missouri, and cancelling all other planned bases. Construction on the two remaining bases continued until 1974, when an additional agreement limited both countries to a single ABM site. The Montana site was abandoned with the main radar partially completed. The remaining base in North Dakota, the Stanley R. Mickelsen Safeguard Complex, became active on 1 April 1975 and fully operational on 1 October 1975. By that time the House Appropriations Committee had already voted to deactivate it. The base was shut down on 10 February 1976. History Nike Zeus An anti-ICBM defensive ABM system was first considered by the US Army in 1955 under the name Nike II. This was essentially an upgraded version of their Nike B surface-to-air missile (SAM) along with dramatically improved radars and computers able to detect the incoming reentry vehicles (RVs) and develop tracking information while still leaving enough time for the interceptor missile to climb to its altitude. Zeus had limited traffic handling capabilities, designed to deal with a small number of attacking missiles arriving over a period of as long as an hour. It was calculated that an attack of only four missiles arriving within one minute would allow one of the warheads to pass by while the system was busy attacking others, making it relatively easy to attack the Zeus base. However, in an era when ICBMs cost about the same as a strategic bomber, such an attack would cost an enormous amount. Through the late 1950s a new generation of much lighter thermonuclear bombs cut warhead weight from in the case of the original Soviet R-7 Semyorka ICBM to perhaps , and further reductions were known to be possible - the US's W47 of the UGM-27 Polaris weighed only . This meant that much smaller rockets could carry these new warheads to the same range, greatly reducing the cost of the missile, making them far cheaper than bombers or any other delivery system. When Nikita Khrushchev angrily boasted that the Soviet Union was producing new missiles "like sausages", the US responded by building more ICBMs of their own, rather than attempting to defend against them with