After Christianization, the Roman Catholic Church and local rulers led German expansion and settlement in areas inhabited by Slavs and Balts, known as Ostsiedlung. During the wars waged in the Baltic by the Catholic German Teutonic Knights; the lands inhabited by the ethnic group of the Old Prussians (the current reference to the people known then simply as the "Prussians"), were conquered by the Germans. The Old Prussians were an ethnic group related to the Latvian and Lithuanian Baltic peoples. The former German state of Prussia took its name from the Baltic Prussians, although it was led by Germans who had assimilated the Old Prussians; the old Prussian language was extinct by the 17th or early 18th century. The Slavic people of the Teutonic-controlled Baltic were assimilated into German culture and eventually there were many intermarriages of Slavic and German families, including amongst the Prussia's aristocracy known as the Junkers. Prussian military strategist Karl von Clausewitz is a famous German whose surname is of Slavic origin. Massive German settlement led to the assimilation of Baltic (Old Prussians) and Slavic (Wends) populations, who were exhausted by previous warfare.

The Expansion of Germany  by the Catholic Church into the areas of the Slavs and balts is know as what?