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 BCE, and later became Noricum, a Roman province, in the mid 1st century CE - an area which mostly encloses today's Austria. In 788 CE, 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. After establishing the First Austrian Republic in 1919, Austria joined Nazi Germany in the Anschluss in 1938. This lasted until the end of World War II in 1945, after which Austria was occupied by the Allies.