The imperialists that Castro is referring to are the Americans.
Castro is asking Khrushchev to attack the US if the US decides to invade Cuba, no matter what the cost.
Castro believes that the United States are determined to destroy them and the the Soviet Union are the only ones who can assist.
I believe that Castro is narcissistic and is obsessed with protecting his country so that he isn't proven weak and is using the Soviet Union as a "eternal guardian."
