how and when did the dukes of brandenburg become kings of prussia
List of rulers of Brandenburg The early rulers came from several different dynasties, but from 1415 Brandenburg and its successor states were ruled by the House of Hohenzollern for over 500 years. From 1618 onward, Brandenburg was ruled in personal union with the Duchy of Prussia. The Hohenzollerns raised Prussia to a kingdom as the Kingdom of Prussia in 1701, and from then on Brandenburg was de facto treated as part of the kingdom even though it was legally still part of the Holy Roman Empire. The titles of Margrave of Brandenburg and Elector of Brandenburg were abolished along with the Holy Roman Empire in 1806, and Brandenburg was formally integrated into Prussia. Despite this, the Prussian kings still included the title "Margrave of Brandenburg" in their royal style. From 1871 to 1918 the Hohenzollerns were also German Emperors.
Unification of Germany Historians debate whether Otto von Bismarck—Minister President of Prussia—had a master plan to expand the North German Confederation of 1866 to include the remaining independent German states into a single entity or simply to expand the power of the Kingdom of Prussia. They conclude that factors in addition to the strength of Bismarck's Realpolitik led a collection of early modern polities to reorganize political, economic, military, and diplomatic relationships in the 19th century. Reaction to Danish and French nationalism provided foci for expressions of German unity. Military successes—especially those of Prussia—in three regional wars generated enthusiasm and pride that politicians could harness to promote unification. This experience echoed the memory of mutual accomplishment in the Napoleonic Wars, particularly in the War of Liberation of 1813–14. By establishing a Germany without Austria, the political and administrative unification in 1871 at least temporarily solved the problem of dualism.