In June 1914 the Archduke Franz Ferdinand, heir to the Habsburg throne, was murdered. This led to World War I. After the war an independent republic called Czechoslovakia was formed with Prague as its capital. Bohemia, Moravia and Slovakia were part of it. In 1938 Hitler invaded the country. It was liberated by Soviet troops in May 1945. However, the communists soon seized power and the country was ruled by communists who had to obey the Soviet Union. The prime minister Alexander Dubček gradually tried to make reforms. This period of time is called the "Prague Spring". In 1968 the Soviet Union sent tanks into Prague to Wenceslas Square to restore their power.