As an upperclassman at Harvard, Kennedy became a more serious student and developed an interest in political philosophy. In his junior year, he made the Dean's List .  In 1940, Kennedy completed his thesis, "Appeasement in Munich", about British participation in the Munich Agreement . The thesis became a bestseller under the title Why England Slept .  In addition to addressing Britain's failure to strengthen its military in the lead-up to World War II, the book also called for an Anglo-American alliance against the rising totalitarian powers. While Kennedy became increasingly supportive of . intervention in World War II, his father's isolationist beliefs resulted in the latter's dismissal as ambassador to the United Kingdom, creating a split between the Kennedy and Roosevelt families. 
However, the greatest crisis of the Kennedy administration was the Cuban Missile Crisis of October 1962. Discovering that the Soviet Union had sent ballistic nuclear missiles to Cuba, Kennedy blockaded the island and vowed to defend the United States at any cost. After several of the tensest days in history, during which the world seemed on the brink of nuclear annihilation, the Soviet Union agreed to remove the missiles in return for Kennedy's promise not to invade Cuba and to remove American missiles from Turkey. Eight months later, in June 1963, Kennedy successfully negotiated the Limited Nuclear Test Ban Treaty with Great Britain and the Soviet Union, helping to ease Cold War tensions. It was one of his proudest accomplishments.