Austria has had much higher naturalization rates than Germany. Two arguments are made based on institutional regime theory and left political power. First, the imperial experiences of the Austro-Hungarian Empire that colonized 11 different nations explains Austria's relative openness, and the monocultural experience of the German Reich that tried to impose German language and culture on partitioned Poland casts light on Austria's open and Germany's rather closed approach to ethnic integration. This first argument covers initial state formation focusing on ethnicity, the Austrian colonization versus German occupation, different ethnicities and languages in the military and bureaucracy, and comparisons involving the partition of Poland and religion in Bavaria. The second argument is a political analysis of legislation concerning how institutionalized regime types and left/green party power influenced the naturalization policies that were enacted into law from 1946 to 2005. The post-World War II analysis shows the positive effects of left/green party power on naturalization, but the institutional regime hypothesis is still necessary to fully explain these differences. In the end, regime differences, and in the later period, left/green party power demonstrate why these two very similar countries have such different naturalization policies.