Of the kingdoms of Germany, Prussia was the largest, most populous, and most important state of the Imperial German Empire. The merger of the Markgraviate of Brandenburg and the Duchy of Prussia, led in 1618 to the formation of the double state, Electorate of Brandenburg and Duchy of Prussia, the latter renamed in 1701 as the Kingdom of Prussia. Prussia was a member state of the German Confederation from 1815 to 1866. After the Prussian-Austrian War, it became the leading member state of the North German Confederation in 1867. It became a member state of the Imperial German Empire in 1871.
Confusion often comes when discussing the kings of Prussia. Fredrick the Great (Fredrick II) ruled from May 1740 - August 1786. He was followed by Friedrich Wilhelm II (August 1786 - November 1797), and Friedrich Wilhelm III (November 1797 - June 1840). It was Friedrich Wilhelm III who led Prussia during the Napoleonic Wars. Friedrich Wilhelm IV was king from June 1840 - January 1861. Wilhelm I became regent for Friedrich Wilhelm IV from October 1858 until the monarch’s death in 1861. Wilhelm I was King of Prussia for the wars against Denmark, and Austria and France. He was declared German Kaiser in 1871 and ruled until 9 March 1888. This was the grandfather of the wartime Kaiser and was declared “Wilhelm the Great” by his grandson. His son, Friedrich III, who had throat cancer and could hardly speak, ascended the throne as the second Kaiser for only 99 days: 9 March 1888 - 15 June 1888. His famous saying roughly translated was: “Learn to suffer without complaining.” The next in line was the third and last Kaiser and King of Prussia, Wilhelm II. He took over 15 June 1888 and abdicated 9 November 1918.
Prussia was a hereditary monarchy in the male line of the house of Hohenzollern with two houses of parliament. The king alone exercised the executive power, had the supreme command of the army, and was head of the protestant church; however, the king shared the legislative power with the Prussian parliament. The legislative assembly or Landtag consisted of two chambers. The consent of both houses, as well as the king, was required before a measure could pass. The chambers had control of the finances and controlled taxes. The upper chamber, Herrenhaus, contained both hereditary and non-hereditary members. The hereditary members were the adult princes of the house of Hohenzollern, the important princes and counts of the old imperial nobility, and the heads of the great territorial nobility. The non-hereditary members were chosen for life by the king from the ranks of the rich landowners and manufacturers. The lower house, or Abgeordnetenhaus, consisted of 433 members, elected for periods of five years by indirect suffrage, by all male citizens who were at least 25. This used the Prussian three-tier system of elections. Based on the election of 1908, if all of the votes had been of equal value, the Social Democrats would have had 113 of 443 seats. With the three-tier system, the SPD won only seven.
It is essential to remember that the Kingdom of Prussia and the Imperial German Empire were separate. The King of Prussia was at the same time Imperial German Emperor, and the Prime Minister of Prussia was also the Imperial Chancellor. The ministries of war and foreign affairs were the same as those of the empire. It was sometimes difficult to determine which hat the individual was wearing. The imperial government administered the customs, postal, and telegraph services. Prussia had seventeen votes in the Bundesrat and sent more than half of the members to the Reichstag. Many individuals were deputies of both the imperial and Prussian parliaments. The Imperial Reichstag and Prussian Landtag were often in session at the same time, and legislation in the Kingdom often took a backseat. The Empire and the Kingdom of Prussia often shared diplomats. A full diplomatic slate was available to the Empire.
Prussia consisted of 12 separate provinces that were internal parts of the Kingdom of Prussia. Frequently they are confused with separate states. This is especially true when you have the Prussian state of Saxony, which is adjacent to the Kingdom of Saxony. The capital of the Kingdom of Prussia was Berlin, the largest city of the empire and the Kingdom had a population of 42.1 million in 1914.
(Hilger, 1905) pg. 115-116
The 12 states were Brandenburg, Hannover, Hessen-Nassau, Ostpreußen, Pommern, Posen, Rheinland, Sachsen, Schleswig-Holstein, Westfalen and Westpreußen.
(Kaiserliches Statistisches Amt, 1914)