In 1225, the Teutonic Knights, a military order of crusading knights, headquartered in the Kingdom of Jerusalem at Acre transferred their operations to the Baltic Sea where Order engaged in numerous armed conflicts until Order's lands came into the hands of a branch of the Hohenzollern family, who already ruled the Brandenburg.
The resulting state, known as Brandenburg-Prussia, commonly known as "Prussia", consisted of geographically disconnected territories in Prussia, Brandenburg, and the Rhineland. During the Thirty Years' War (1618–1648), armies repeatedly marched across the territories so Hohenzollerns had to build a powerful military to protect disconnected lands.
"Prussia" developed one of the most powerful armies in Europe. Mirabeau said: "Prussia, is not a state with an army, but an army with a state." More than 20,000 Protestant refugees from Salzburg settled in thinly populated eastern Prussia.
Prussia engaged in wars with Poland, Lithuania, numerous German States, Habsburg Austria, France, and Russia proving Prussia's status as one of the great powers of Europe. By 1813, Prussia could mobilize almost 300,000 soldiers. Prussian troops contributed crucially in the Battle of Waterloo - the final victory over Napoleon. Prussia invited the immigration of Protestant refugees (especially Huguenots).
For protestants, Prussia was a safe haven in much the same way that the United States welcomed immigrants seeking freedom in the 19th century. Frederick the Great, the first "King of Prussia" introduced a general civil code, abolished torture and established the principle that the Crown would not interfere in matters of justice. He promoted an advanced secondary education which prepares the brightest pupils for university studies. The Prussian education system was emulated in various countries, including the United States.
The first half of the 19th century saw a prolonged struggle between those who wanted a united Germany and others who wanted to maintain Germany as a patchwork of independent, monarchical states with Prussia and Austria competing for influence. In 1862 Prussian King Wilhelm I appointed Otto von Bismarck as Prime Minister. Bismarck guided Prussia through a series of wars resulting in a formation of the North German Confederation that united all German-speaking peoples, excluding Austria, which remained connected to non-German territories. On 18 January 1871, William was proclaimed "German Emperor".
World War I ended Prussia’s supremacy. The abolition of the political power of the aristocracy transformed Prussia into a region strongly dominated by the left-wing of the political spectrum. Prussia lost territories and became a Land under the Weimar Republic. After the rise to power of Adolf Hitler in 1933, the Prussian constitution was set aside and the legislature abolished.
World War II led to the abolition of Prussia with most the land ceded over to Poland. The German population was expelled and fled to the Western occupation zones. The number of casualties is estimated at 2 to 4 million, including those who fled during the last months of the war. 25 February 1947, Prussia was officially proclaimed to be dissolved.