Once the center of power for the large Austro-Hungarian Empire, Austria was reduced to a small republic after its defeat in World War I. Following annexation by Nazi Germany in 1938 and subsequent occupation by the victorious Allies in 1945, Austria's status remained unclear for a decade. A State Treaty signed in 1955 ended the occupation, recognized Austria's independence, and forbade unification with Germany. A constitutional law of that same year declared the country's "perpetual neutrality" as a condition for Soviet military withdrawal. This neutrality, once ingrained as part of the Austrian cultural identity, has been called into question since the Soviet collapse of 1991 and Austria's entry into the European Union in 1995. A prosperous country, Austria entered the European Monetary Union in 1999.