The origins of Austria date back to the time of the Roman Empire when a Celtic kingdom was conquered by the Romans in approximately 15 B.C. and later became Noricum, a Roman province, in the mid-1st century AD?an area which mostly encloses today's Austria. In 788 A.D., the Frankish king Charlemagne conquered the area and introduced Christianity. Under the native Habsburg dynasty, Austria became one of the great powers of Europe. In 1867, the Austrian Empire was reformed into Austria-Hungary. The Austro-Hungarian Empire collapsed in 1918 with the end of World War I. The First Austrian Republic was established in 1919. In the 1938 Anschluss, Austria was occupied and annexed by Nazi Germany. This lasted until the end of World War II in 1945, after which Austria was occupied by the Allies and its former democratic constitution was restored. In 1955, the Austrian State Treaty re-established Austria as a sovereign state, ending the occupation. In the same year, the Austrian Parliament created the Declaration of Neutrality which declared that the country would become permanently neutral.