The SALT Treaty with the USSR allowed each country to put anti-ballistic systems in one place per country. The USA opted to ring the ICBM fields to have them available for retaliation against a first strike.
The USSR, who swore they would NEVER be the first to launch in a nuclear war opted to ring Moscow with ABM. Why the difference? Russia had no need to protect their missile fields as they would be emptied out in a first strike. So much for NEVER being the first to launch.
The USA opted to ring the ICBM fields to have them available for retaliation against a first strike.
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Nope...there was one ABM system that was built but never operated for the missile complex that was at Grand Forks AFB. All of the Minuteman missiles are gone and the mission closed.
The remaining 400 (out of the 1,000 once deployed) Minuteman missiles are located at Malmstrom AFB (Great Falls, MT), Minot AFB ND and F.E. Warren AFB (Cheyenne, WY).
That’s it folks...and it’s all in the public domain and unclassified.