The leaders of the Soviet Union were committed under the principles of Marx and Lenin to spread Communism throughout the world. They had by far the most powerful land army in Europe and Asia after conquering Germany, and they seemed to be in an excellent position to foment revolutions in every country or colony that had a large percentage of impoverished and exploited people. They became politically involved with unrest and revolution everywhere, even in the United States. The factors that held them back were the destruction and population loss they have incurred in the terrible battle with Germany and also the advantage the United States and its allies held in sea power, air power, and sole possession of the atomic bomb for the first four or five years after World War II ended. The United States was determined to resist the spread of communism. They had the Soviet Union surrounded with air bases and missile bases, and it was obvious that they could deliver atomic bombs to most Soviet cities if a shooting war broke out. Even when the Soviets developed their atomic bomb, they had no effective way of dropping it on American cities; and by that time the U.S. had developed the hydrogen bomb. The Soviets only had long-range missiles which might not be accurate. Meanwhile both sides continued to improve and stockpile atomic weapons until both had enough to destroy the entire planet, and it seemed inevitable that one side or the other would start a nuclear holocaust. The Soviets involved themselves on Castro's side in the Cuban Revolution, turning it into a communist revolution and a Marxist-Leninist communist state. When Khruschev tried to install short-range atomic missiles in Cuba, only sixty miles from the U.S. mainland, it led to the Cuban Missile Crisis with the imminent threat of an outbreak of World War III. However, the Soviets backed down when President Kennedy threatened to sink the Russian ship bringing missiles and atomic warheads to Cuba. An American invasion of Cuba also seemed likely. This was the turning point in the Cold War. The Soviet system could not continue to exist without worldwide expansion. It eventually collapsed, leaving the U.S. the only superpower in the world until the present day.