Short answers, pulled from the story.
The Soviet Union established its first embassy in Havana during World War II in 1943. This diplomatic opening occurred when Cuban diplomats under President Fulgencio Batista visited Moscow the same year.
Khrushchev agreed on a deployment plan for nuclear weapons in May 1962 primarily due to Castro's fears of American invasion. The crisis strained relations because Castro was not consulted throughout the negotiations and felt angered by the unilateral Soviet withdrawal of missiles and bombers without his input.
Cuba joined the Council for Mutual Economic Assistance or Comecon in 1972 as an economic organization dominated by the Soviet Union. This membership allowed it to obtain oil at highly favorable rates and enabled the Soviets to invest US$1.7 billion on construction and remodeling of Cuban factories between 1976 and 1980.
Mikhail Gorbachev came to power in March 1985 introducing reforms that would eventually undermine the alliance. His policies of perestroika and glasnost created ideological conflict with the Cuban regime, leading to reduced subsidies and widespread shortages.
The dissolution of the Soviet Union occurred in December 1991 and had an immediate devastating effect on Cuba. Trade with the former Soviet bloc declined by more than 90% following the shift to world market prices, causing oil imports to drop from 13 million tons in 1989 to about 3 million tons in 1993.