The Elector changed sides three times, and as a result Protestant groups of Evangelical Lutheran / Reformed / Calvinist and opposing Roman Catholic armies swept the lands back and forth, killing, burning, seizing men and confiscating any food or useful supplies.

It is estimated that upwards of half the population of Central Europe / Germany of the late Medieval / Middle Ages period were killed or dislocated.

[8] The Brandenburg-Saxon antagonism rendered the defense of the circle ineffective, and it was subsequently overrun by Albrecht von Wallenstein during the Thirty Years' War.

[8] While George William had claimed neutrality before, the presence of Wallenstein's army forced him to join the Catholic-Imperial camp in the Treaty of Königsberg (1627) and accept garrisons.

[11] The character of the young elector had been stamped by his Calvinist nurturer Calcum, a long stay in the Dutch Republic during his grand tour, and the events of the war, of which a meeting with his uncle Gustavus Adolphus of Sweden in Pomerania was among the most impressive.

[12] Whether or not Frederick William concluded a truce and neutrality agreement with Sweden is disputed: while a relevant 1641 document exists, it was never ratified and has repeatedly been described as a falsification.

[17] This was primarily due to French efforts to counterbalance the power of the Habsburg emperor by strengthening the Hohenzollern, and while Frederick William valued these territories lower than Western Pomerania, they became step-stones for the creation of a closed, dominant realm in Germany in the long run.

[19] In June 1651, Frederick William broke the provisions of the Peace of Westphalia by invading Jülich-Berg, bordering his possessions in Cleves-Mark at the lower Rhine river.

[26] While military confrontations were avoided and the Brandenburg-Prussian army was primarily occupied with stealing cattle (hence the name), it considerably lowered Frederick William's reputation.

[28] In turn, he confirmed several privileges of the knights, including tax exemption, assertion of jurisdiction and police powers on their estates (Patrimonialgerichtsbarkeit) and the upholding of serfdom (Leibeigenschaft, Bauernlegen).

[7] Continued pressure on Charles X Gustav resulted in him conceding full sovereignty in Ducal Prussia and Ermland to Frederick William by the Treaty of Labiau in November to ensure the maintenance of the alliance.

[36] When the anti-Swedish coalition however gained the upper hand, Frederick William changed sides when Polish king John II Casimir Vasa confirmed his sovereignty in Prussia, but not in Ermland, in the Treaty of Wehlau-Bromberg in 1657.

[39] When the Holy Roman Empire declared war on France, a so-called Reichskrieg, Brandenburg-Prussia again changed sides and joined the imperial forces.

[44] For this plan to work out, Poland-Lithuania had to first conclude her war against the Ottoman Empire, which French diplomacy despite great efforts failed to achieve.

[50] For this alliance, Frederick William relinquished rights on Silesia in favor of the Habsburgs, and in turn received the Silesian County of Schwiebus which bordered the Neumark.

In the mid-16th century, the margraves of Brandenburg had become highly dependent on the estates (counts, lords, knights and towns, no prelates due to the Protestant Reformation in 1538).

[29] The conclusion of the Second Northern War had strengthened the elector politically, enabling him to reform the constitution of Cleves and Mark in 1660 and 1661 to introduce officials loyal to him and independent of the local estates.

[29] Due to Frederick William's reforms, the state income increased threefold during his reign,[31] and the tax burden per subject reached a level twice as high as in France.

The "Great Elector" had spent part of his childhood at the Pomeranian court and port cities of Wolgast (1631–1633) and Stettin (1633–1635), and afterwards studied at the Dutch universities of Leyden and The Hague (1635–1638).

[66] After the Thirty Years' War, Frederick William tried to acquire finances to rebuild the country by participating in oversea trade, and attempted to found a Brandenburg-Prussian East Indies Company.

[67] He engaged former Dutch admiral Aernoult Gijsels van Lier as advisor and tried to persuade the Holy Roman Emperor and princes of the empire to participate.

[67] As Frederick William was unable to raise this sum, he asked several people and Hanseatic towns to invest in the project, but since none of these were able or willing to give sufficient money, the treaty with Denmark was nullified in 1653.

[69] He engaged Dutch merchant and shipowner Benjamin Raule as his advisor, who after a first personal meeting with Frederick William in 1675 settled in Brandenburg in 1676[69] and became the major figure of Brandenburg-Prussia's emerging naval and colonial enterprise.

The Brandenburg-Prussian navy was established from ten ships which Frederick William leased from Raule, and achieved first successes in the war against Sweden supporting the siege of Stralsund and Stettin and the invasion of Rügen.

[72] While Emden was not part of Brandenburg-Prussia, the elector owned a nearby castle, Greetsiel, and negotiated an agreement with the town to maintain a garrison and a port.

[73] Since the crew of the Wappen von Brandenburg sold a barrel of brandy to Africans in a territory claimed by the Dutch West Indies Company, the latter confiscated the ship in March.

[73] To facilitate the colonial expeditions, the Brandenburg African Company was founded on 7 March 1682,[75] initially with its headquarters in Berlin and its shipyards in Pillau, since 1683 in Emden.

[75] In July 1682, an expedition under East Prussian Otto Friedrich von der Groeben was sent to Guinea to erect the fortress Großfriedrichsburg.

[77] On 24 February 1684, another treaty with indigenous chiefs was signed that allowed the erection of a second fort in nearby Accada (now Akwida),[78] named Dorotheenschanze after Frederick William's second wife.

[86] Brandenburg-Prussia was allotted an area near the capital city Charlotte Amalie, called Brandenburgery, and other territories named Krum Bay and Bordeaux Estates further west.