For those too young to have learned, or old enough to have forgotten, the Cold War was nasty, brutish, and long. Historians disagree vehemently about its origins, about both who was responsible and when it began. But it was under way by 1947 at the latest, driven partly by conflicting ideologies among the victorious allies in World War II, partly by conflicting economic and political interests, and partly by a host of lesser considerations, including personalities, misunderstandings, and much else.
The Cold War fault lines derived from World War II. Its main theaters, Europe and East Asia, were the main theaters of World War II. On one side stood the Soviet Union of Joseph Stalin and the East European countries the Red Army had liberated (or conquered) in 1944–5 in the savage war with Germany. On the other side stood the United States and Britain, supported by dozens of allied countries, notably those liberated and occupied by Anglo-American forces in the last months of World War II. In East Asia, the defeat of Japan left a divided China, which embarked on a civil war between Communists, often but not always supported by Stalin, and nationalists, often but not unconditionally supported by the United States.
From 1948 to 1962, the Cold War featured a series of crises that threatened to convert it into World War III. The biggest shift in the balance of power came in 1949, when the Chinese communists under Mao Zedong's leadership won the civil war and drove the nationalists to the island of Taiwan, and when the Soviet Union successfully exploded its first nuclear weapon.