Prussia was a German state that existed from 1525 through the end of WWII, after which time it was formally abolished as a distinct political entity. The Kingdom of Prussia had a prominent role in Early Modern European history and was considered a great power and was notable for its military tradition and its role in German unification. At its height, Prussian controlled much of the North German Confederation and included large areas of what is today Germany and Poland. It was ruled by the house Hohenzollern until the 1918 abdication of Emperor William II of Germany, who was also King of Prussia.
The later state of Prussia was largely centered around Berlin and Brandenburg but was originally based in the historical region of the Southeastern Baltic coast known as Prussia, and in particular, the German state of East Prussia. Named for the Baltic-speaking peoples of the same name who lived there, the region of Prussia was colonized by the Teutonic order and eventually Germanized from the Medieval period onward. The name “Prussia” has fallen into disuse following the transfer of the region to Poland and the USSR after WWII and the subsequent expulsion of the German-speaking population.
On AH.com, East Prussia and the prospect of a rump state in its place has received some attention.