East Prussia (German: Ostpreußen, Polish: Prusy Wschodnie, Russian: Vostochnaya Prussiya) was a province of the Kingdom of Prussia, which roughly corresponded to what used to be the Duchy of Prussia. The Duchy of Prussia had been a fief of Poland, created as such in 1525. The second Treaty of Thorne (1466), which ended the Thirteen Years War between Poland and the Teutonic Order of Knights had led to the partitioning of the land known as Prussia into Royal Prussia (the West, ruled by the Polish crown) and the Eastern part, which the Order retained control over. However, in 1525 Grand Master Albert of Brandenburg, a Hohenzollern, converted to Lutheranism and converted east Prussia into a Duchy under Polish auspices, so that he might avoid armed conflict with Poland. Thus East Prussia became the first Protestant state.
In 1660 the Second Northern War between Brandenburg (also ruled by a Hohenzollern), Sweden and Poland led to the Treaty of Welawa, that resulted in Frederick William I gaining full sovereignty over Ducal Prussia, creating Brandenburg-Prussia (the Treaty also stipulated that when Hohenzollern rule in the state expired, it would revert to Polish rule). After the first partition of Poland in 1772, Prussia assumed control of Royal Prussia (henceforth West Prussia) and part of the province of Warmia became part of East Prussia. Along with the rest of Prussia, East Prussia became part of the German Empire which was created in 1871. After World War I and the abdication of Emperor Wilhelm II in 1918, East Prussia should have reverted back to Polish control under the Treaty of Welawa, but this did not happen. Instead, East Prussia became an exclave of Germany that contained over a million Germans.
The Treaty of Versailles, which redrew the map of much of Eastern and Central Europe following the downfall of the German and Ottoman Empires (the Romanov Empire fell as well, but was reborn as the Soviet Union), separated East Prussia from the rest of Germany by the creation of the Polish Corridor. The Polish Corridor was a strip of old Royal Prussian territory that was at times no more than 40km wide, but gave Poland access to the Baltic Sea - as promised by Woodrow Wilson in his Fourteen Points. Only 30% of the 2.5 million East Prussians used the Polish language, although the inhabitants of the Corridor were mostly Polish. But there was a large German minority, and this angered the pan-Germanism of Adolf Hitler.
Not only did Hitler's ideology demand that all Germans be within the borders of his nation, but he also had a more pragmatic reason for being angry at the existence of the Polish Corridor - he wanted to get troops and equipment into East Prussia. He thus demanded that he be given a corridor of land through the Polish Corridor, but he was refused. Poland received assurance against aggression from France and Great Britain, but Hitler invaded Poland and World War II began. Hitler annexed the Polish Corridor and East Prussia became part of Greater Germany. Then, when the Soviets came (killing and driving away hundreds of thousands of ethnic Germans), the northern part of East Prussia was annexed by the Soviet Union. The old capital city of Königsberg was renamed Kaliningrad to honour Mikhail Ivanovich Kalinin, who had taken part in both Russian Revolutions and was then President of the USSR. The southern part of East Prussia reverted to Poland, and is now the administrative district of Warmia i Mazury. The northern part is now an exclave of the Russian Federation, with Lithuania, Belarus and Latvia lying between it and the rest of its country.