The history of Vienna has been long and varied, beginning when the Roman Empire created a military camp in the area covered by Vienna's city center. In 1440 Vienna became the resident city of the Habsburg dynasty, who ruled the Austria for more than 500 years. City eventually grew to become the de facto capital of the Holy Roman Empire. In 1804, during the Napoleonic Wars, Vienna became the capital of the newly-formed Austrian Empire. The city continued to play a major role in European and world politics, including hosting the Congress of Vienna in 1814/15, where new borders in Europe were established. At the beginning of 20th century, Vienna was the capital of Austro – Hungarian Empire, country that existed until end of the First World War. Just before collapse, Vienna was the city of more than 50 million people. In March 1938 Nazi Germany occupied and annexed Austria in a process known as the “Anschluss”. Adolf Hitler was rapturously received in Vienna by large crowds of admirers and famously gave a speech at Heldenplatz in which he welcomed his homeland into the Reich. After second world war, similar to Berlin, Vienna was also divided into five occupation zones between the Soviet Union, the United States, the United Kingdom, France, and with the first district (city center) being patrolled by all four. On May 15, 1955, the country regained its political independence and sovereignty. The four-power control of Vienna lasted until the Austrian State Treaty was signed in May 1955. That year, after years of reconstruction and restoration, the State Opera and the Burgtheater, both on the Ring road, reopened to the public. Vienna has regained much of its former international stature by hosting international organizations, such as the United Nations, the International Atomic Energy Agency, the Organization of Petroleum Exporting Countries, and the Organization for Security and Cooperation in Europe. Now it is a home of about 2 million people.