Status: Operational 1964.
Program 437 was America's second operational anti-satellite system. It was launched on sub-orbital trajectories by Thor LV-2D's operated by the U.S. Air Force from Johnson Atoll. Perhaps not coincidentally, Thor had been used for space nuclear tests from the same launch site in the early 1960's. These tests that inadvertently led to discovery of the extreme damage to electronics that could be caused by EMP (electromagnetic pulse) from a high altitude nuclear detonation. The Program 437 Thor could hit satellites up to 700 km altitude using a Mk. 49 nuclear warhead with an 8 km kill radius. The first flight was February 1964, and the system was declared operational in June 1964. The system had a two week reaction time - the missiles and warheads were stored in kits at Vandenberg and had to be deployed to the atoll.
The limitations of a nuclear-tipped ASAT system were apparent - using nuclear warheads just to get a satellite would be destabilizing in a non-nuclear war scenario. Furthermore the EMP effect could damage a large number of your own satellites. The Program 437 system was put 'on standby' in 1970 and canceled in 1972 when a hurricane damaged the guidance computers on the atoll beyond economic repair.
In 1965-1966 four Program 437 Thors were launched with 'Alternate Payloads' for satellite inspection. This was evidently an elaboration of the system to allow visual verification of the target before destroying it. These flights may have been related to the late 1960's Program 922, a non-nuclear version of Thor with infrared homing and a high explosive warhead.
The SALT-1 Treaty of May 1972 prohibited 'interference with national means of verification' which meant that ASAT's were not allowed, by treaty, to attack Russian spy satellites. While this did not explicitly prohibit development of ASAT's, further development would be met by Soviet protests and opposition by certain political groups in the US.
After the loss of Program 437 capability, the USAF moved on to air-launched conventional warhead ASAT's, first with project SPIKE for launch from an F-106, culminating in the F-15 ASAT missile of the 1980's.