Other contributors have said "What were the soviets demands in the cuban missile crisis?" is the same question as "Who caused the Cuban missile crisis?". If you believe that these are not asking the same thing and should be answered differently, click here.
The Cuban missile crises was caused by the Soviet Union, who was allied with Cuba. The Soviet Union dispatched missiles to Cuba to strike the U.S. mainland and deal a crippling blow to the U.S. military and morale.