Modern era
During the Napoleonic era the Greater Polish territories and the Chełmno Land formed part of the Duchy of Warsaw following the Treaties of Tilsit, and Danzig was granted a status of a Free City. However, after the Congress of Vienna, the Polish duchy was again partitioned between Russia and Prussia, with Prussia annexing the Free City and the Chełmno Land into the reconstituted West Prussia.
Though the Kingdom of Prussia was a member of the German Confederation established by the Congress of Vienna as a replacement for the dissolved Holy Roman Empire from 1815 to 1866, only those of the territories of the kingdom that were previously included in the HRE as well as the Lauenburg and Bütow Land and the former Starostwo of Draheim (both integrated into the Province of Pomerania) became part of the Confederation, while the Grand Duchy of Posen (later demoted to an ordinary Province of Posen following the failed Greater Poland uprising (1848)), the Provinces of West Prussia and East Prussia (merged in the years 1829 to 1878 to form a single Province of Prussia) remained outside of the German Confederation (thus of Germany) until the creation of the North German Confederation in 1866 at the start of the unification of Germany.
As agreed upon in the Treaty of Versailles, East Prussia, minus the Memelland, expanded by addition of the Regierungsbezirk Westpreussen (the only part of former West Prussia containing originally Prussian territory) remained within the Free State of Prussia, a successor of the Kingdom of Prussia and a constituent part of the German Weimar Republic, following the 1920 East Prussian plebiscite. The democratic government of the Free State was removed as a result of the 1932 Prussian coup d'état, which also facilitated the Nazi takeover of government. In the March 1933 German federal election, the last pre-war German elections, the local population of East Prussia voted overwhelmingly for Adolf Hitler's Nazi Party.
After the 1939 German ultimatum to Lithuania, the Klaipėda region was integrated again into East Prussia. During World War II, the Polish ethnic minorities of Catholic Warmians and Powiślans, as well as Lutheran Masurians were persecuted by the Nazi German government, which wanted to erase all aspects of Polish culture and Polish language in Warmia, Masuria and Powiśle. The Jews who remained in East Prussia in 1942 were shipped to concentration camps, including Theresienstadt in occupied Czechoslovakia, Kaiserwald in occupied Latvia, and camps in Minsk in occupied Byelorussian Soviet Socialist Republic.
The Pomeranian Voivodeship was occupied and illegally annexed by Nazi Germany during the invasion of Poland in 1939, as well as renamed Reichsgau Danzig-Westpreussen, with numerous German atrocities against the local population. Nazi Germany enlarged the territory of the province of East Prussia through annexation into it of parts of Northern Masovia under the name of Regierungsbezirk Zichenau, as well as of the Suwałki Region, both referred to as South East Prussia, in relation to the New East Prussia of the times of Polish Partitions (see above). At the same time, the Regierungsbezirk Westpreussen was separated from East Prussia and integrated into Reichsgau Danzig-Westpreussen.
The Soldau concentration camp and Hohenbruch concentration camp were operated in the region mostly for Polish prisoners, as well as several subcamps of the Stutthof concentration camp, and several prisoner-of-war camps for Allied POWs, including Polish, Belgian, British, French, Dutch, Serbian, Italian, Soviet, Australian, Canadian, New Zealander, South African, such as Stalag I-A, Stalag I-B, Stalag I-C, Stalag I-D, Stalag XX-B, Stalag Luft VI and Oflags 52, 53, 60 and 63 with numerous forced labour subcamps. Many expelled Poles from German-occupied Poland were enslaved by the Germans as forced labour in the region. There was also a camp for Romani people in Königsberg (see Romani Holocaust).
The Polish resistance movement was active in the region, with activities including distribution of Polish underground press, infiltration of the German arms industry, sabotage actions, executions of Nazis, theft of German weapons, ammunition and equipment, smuggling data on German concentration camps and prisons, and organization of transports of POWs who escaped German POW camps via the ports of Gdańsk and Gdynia to neutral Sweden.
Beginning in 1944 with the East Prussian offensive of Soviet troops, the German-speaking population was evacuated.