Founded around 500 BC, Vienna was originally a Celtic settlement. In 15 BC, Vienna became a Roman frontier city (Vindobona) guarding the Roman Empire against Germanic tribes to the north.
Vienna came under threat from the Mongolian Empire that stretched over much of present day Russia and China in the 1200s. However, due to the death of an important leader, the Mongolian Empire deteriorated and Europe was freed from the threat.
During the Middle Ages, Vienna was home to the Babenberg Dynasty and in 1440 AD became residence city of the Habsburg dynasties from where Vienna eventually grew to become the capital of the Holy Roman Empire and a cultural centre for arts and science, music and fine cuisine. In the 16th and 17th centuries, the Ottoman armies were stopped twice outside Vienna (see Siege of Vienna, 1529 and Battle of Vienna, 1683).
In 1804, Vienna became capital of the Austrian Empire and continued to play a major role in European and World politics, including hosting the 1814 Congress of Vienna. After the Austro-Hungarian Compromise of 1867 Vienna remained the capital of what was then the Austro-Hungarian Empire. During the latter half of the 19th century the city developed what had previously been the bastions and glacis into the Ringstraße, a major prestige project. Former suburbs were incorporated, and the city of Vienna grew drastically.
In 1918, after World War I, Vienna became capital of the First Austrian Republic. During the 1920s and 1930s it was a bastion of Socialism in Austria, and became known as "Red Vienna." The city was stage to the Austrian Civil War of 1934, when Chancellor Engelbert Dollfuss sent the Army to shell civilian housing occupied by the socialist militia. In 1938, after a triumphant entry into Austria, Adolf Hitler famously spoke to the Austrian people from the balcony of the Neue Burg, a part of the Hofburg at the Heldenplatz. Between 1938 (Anschluß) and the end of the Second World War, Vienna lost its status as a capital to Berlin.
In 1945, the Vienna Offensive was successfully launched by the Soviets against the Germans holding Vienna. The city was besieged for about two weeks before it fell to the Soviets. After 1945, Vienna again became the capital of Austria. It was initially divided into four zones by the 4 Powers and was governed by the Allied Commission for Austria. During the 10 years of foreign occupation Vienna became a hot-bed for international espionage between the Western and Eastern blocs.
In the 1970s Austrian Chancellor Bruno Kreisky inaugurated the creation of the Vienna International Centre, a new area of the city created to host international institutions. Vienna has regained a part of its former international relevance by hosting such international organizations as the United Nations (UNIDO, UNOV, CTBTO and UNODC), the International Atomic Energy Agency, the Organization of Petroleum Exporting Countries, and the Organization for Security and Cooperation in Europe.