In 1964, a KH-4A Corona satellite spotted a site in Tallinn, Soviet Estonia under construction. The discovery lead the Americans to believe that the Soviets are setting up an anti-ballistic missile (ABM) system at the site.
George Chernilevsky [CC BY-SA 3.0], via Wikimedia Commons
It was actually a S-200 air defense missile system but the policy makers in Washington decided to adopt the Multiple Independent Reentry Vehicle (MIRV) technology for American nuclear missiles in order to overwhelm any ABM system put in place.