1979: Presidents Carter and Brezhnev signed SALT II Treaty.
Six months after the signing, the Soviet Union invaded Afghanistan, and in September of the same year, the United States discovered that a Soviet combat brigade was stationed in Cuba. (Although President Carter claimed this Soviet brigade had only recently been deployed to Cuba, the unit had been stationed on the island since the Cuban Missile Crisis of 1962.) In light of these developments, the treaty was never ratified by the United States Senate. Its terms were, nonetheless, honored by both sides until it expired.
Photo credit: Bill Fitz-Patrick via Wikipedia