William I, or in German Wilhelm I (full name: "William Frederick Louis of Hohenzollern", German: "Wilhelm Friedrich Ludwig von Hohenzollern" , 22 March 1797 – 9 March 1888), of the House of Hohenzollern was King of Prussia from 2 January 1861 and the first German Emperor from 1 January 1871 to his death, the first Head of State of a united Germany.
 Under the leadership of William and his Minister President Otto von Bismarck, Prussia achieved the unification of Germany and the establishment of the German Empire.
 Despite his long support of Otto von Bismarck as Minister President, William held strong reservations about some of Bismarck's more reactionary policies, including his anti-Catholicism and tough handling of subordinates.
 Contrary to the domineering Bismarck, William was described as polite, gentlemanly and, while a staunch conservative, more open to certain classical liberal ideas than his grandson Wilhelm II.
Frederick II (German: "Friedrich" ; 24 January 171217 August 1786) was King of Prussia from 1740 until 1786, the longest reign of any Hohenzollern king.
 His most significant accomplishments during his reign included his military victories, his reorganization of Prussian armies, his patronage of the arts and the Enlightenment in Prussia, and his final success against great odds in the Seven Years' War.
 Frederick was the last titled King in Prussia and declared himself King of Prussia after achieving full sovereignty for all historical Prussian lands.
 Prussia had greatly increased its territories and became a leading military power in Europe under his rule.
 He became known as Frederick the Great ("Friedrich der Große ") and was affectionately nicknamed "Der Alte Fritz " ("Old Fritz") by the Prussian and later by all German people.
The monarchs of Prussia were members of the House of Hohenzollern who were the hereditary rulers of the former German state of Prussia from its founding in 1525 as the Duchy of Prussia.
 The Duchy had evolved out of the Teutonic Order, a Roman Catholic crusader state and theocracy located along the eastern coast of the Baltic Sea.
 The Teutonic Knights were under the leadership of a Grand Master, the last of whom, Albert, converted to Protestantism and secularized the lands, which then became the Duchy of Prussia.
 The Duchy was initially a vassal of the Kingdom of Poland, as a result of the terms of the Prussian Homage whereby Albert was granted the Duchy as part of the terms of peace following the Prussian War.
 When the main line of Prussian Hohenzollerns died out in 1618, the Duchy passed to a different branch of the family, who also reigned as Electors of Brandenburg in the Holy Roman Empire.
 While still nominally two different territories, Prussia under the suzerainty of Poland and Brandenburg under the suzerainty of the Holy Roman Empire, the two states are known together historiographically as Brandenburg-Prussia.
 Following the Second Northern War, a series of treaties freed the Duchy of Prussia from any vassalage to any other state, making it a fully sovereign Duchy in its own right.
 This complex situation (where the Hohenzollern ruler of the independent Duchy of Prussia was also a subject of the Holy Roman Emperor as Elector of Brandenburg) laid the eventual groundwork for the establishment of the Kingdom of Prussia in 1701.
 For diplomatic reasons, the rulers of the state were known as the King in Prussia from 1701 to 1772; largely because they still owed fealty to the Emperor as Electors of Brandenburg, the "King in Prussia" title (as opposed to "King of Prussia") avoided offending the Emperor.
 As the Prussian state grew through several wars and diplomatic moves throughout the 18th century, it became apparent that Prussia had become a Great Power that did not need to submit meekly to the Holy Roman Empire.
 By 1772, the pretense was dropped, and the style "King of Prussia" was adopted.
 Thus it remained until 1871, when in the aftermath of the Franco-Prussian War, the King of Prussia Wilhelm I was crowned German Emperor.
 From that point forward, though the Kingdom of Prussia retained its status as a constituent state of the German Empire, all remaining Kings of Prussia also served as German Emperor, and that title took precedence.
Prince Albert of Prussia (Frederick Henry Albert; German: "Friedrich Heinrich Albrecht" ; 4 October 1809, in Königsberg – 14 October 1872, in Berlin) was a Prussian colonel general.
 Albert was the fifth son and youngest child of King Frederick William III of Prussia and Louise of Mecklenburg-Strelitz.
 His parents had fled to East Prussia after the occupation of Berlin by Napoleon.
 Two of Albert's elder brothers were Frederick William IV, King of Prussia from 1840 till 1861, and William I, King of Prussia from 1861 to 1888 and German Emperor from 1871 until 1888.
The state of Prussia had its origins in the separate lands of the Margraviate of Brandenburg and of the Duchy of Prussia.
 The Margraviate of Brandenburg developed from the medieval Northern March of the Holy Roman Empire, passing to the House of Hohenzollern in 1415.
 The Duchy of Prussia originated in 1525 when Albert of Brandenburg-Ansbach, a member of a cadet branch of the Hohenzollerns, secularized the eastern lands of the Teutonic Knights as a Polish fief.
 Prince-elector John Sigismund, Elector of Brandenburg, inherited the Duchy of Prussia in 1618, thus uniting Brandenburg and Prussia under one ruler in a personal union; the Elector's state became known as Brandenburg-Prussia.
 The Kingdom of Prussia formed when Elector Frederick III assumed the title of Frederick I, King in Prussia, on 18 January 1701.
Frederick Henry Louis (German: "Friedrich Heinrich Ludwig" ) (18 January 1726 – 3 August 1802), commonly known as Henry ("Heinrich "), was a Prince of Prussia and the younger brother of Frederick the Great.
 He also served as a general and statesman, leading Prussian armies in the Silesian Wars and the Seven Years' War, having never lost a battle in the latter.
 In 1786, he was suggested as a candidate for a monarch for the United States.
Frederick I (German: "Friedrich I" ) (11 July 1657 – 25 February 1713), of the Hohenzollern dynasty, was (as Frederick III) Elector of Brandenburg (1688–1713) and Duke of Prussia in personal union (Brandenburg-Prussia).
 The latter function he upgraded to royalty, becoming the first King in Prussia (1701–1713).
 From 1707 he was in personal union the sovereign prince of the Principality of Neuchâtel (German: "Fürstentum Neuenburg" ).
 He was also the paternal grandfather of Frederick the Great.
Hans Hermann von Katte (28 February 1704 – 6 November 1730) was a Lieutenant of the Prussian Army and the friend of the future King Frederick II of Prussia, who was at the time the Crown Prince.
 He was executed by Frederick's father King Frederick William I of Prussia when Frederick II plotted to escape from the Kingdom of Prussia to the Kingdom of Great Britain.
 Some believe that Frederick intended to defect to the service of George II of Great Britain (Frederick William's maternal first cousin and Frederick's maternal uncle) and possibly return to Prussia to depose Frederick William.
Frederick Henry Charles, Prince of Prussia (German: "Friedrich Heinrich Karl" , 30 December 1747 – 26 May 1767) was the second son of Prince Augustus William, the brother of Frederick the Great.
 His older brother was Frederick William II of Prussia.
Princess Marie of Prussia ("Marie Elisabeth Louise Frederika of Prussia"; 14 September 1855, Marmorpalais, Potsdam – 20 June 1888, Dresden), was a princess of the House of Hohenzollern.
 She was the daughter of Prince Frederick Charles of Prussia and later became second wife of Prince Henry of the Netherlands then the first wife of Prince Albert of Saxe-Altenburg.
 She was also the great-niece of Wilhelm I, German Emperor.

Question: Frederick Henry Louis, was a Prince of Prussia and the younger brother of Frederick the Great, was King of Prussia from 1740 until which year, the longest reign of any Hohenzollern king?
Answer: 1786