In exchange for military aid in the Second Northern War and the return of Ermland (Ermeland, Warmia) to Poland, the Polish king granted the Hohenzollern dynasty of Brandenburg hereditary sovereignty in the Duchy of Prussia, pawned Draheim (Drahim) and Elbing (Elbląg) to Brandenburg and handed over Lauenburg and Bütow Land to the Hohenzollerns as a hereditary fief.

[4] After fighting along with the Swedish Army in 1656, most prominently at the Battle of Warsaw, Hohenzollern Frederick William I was willing to abandon his ally when the war had turned against them and signalled his willingness to change sides if Polish King John II Casimir Vasa would grant him similar privileges as previously Swedish King Charles X Gustav, conditions that were negotiated in Wehlau (Welawa, now Znamensk) and Bromberg (Bygost, Bydgoszcz).

[6] On 3 November 1656, the Truce of Vilna had promised Alexis of Russia's election as a successor on the Polish throne at the next diet in return for halting his offensive in Poland–Lithuania and to fight Sweden instead.

[7] In the Grand Duchy of Lithuania, there was support for the treaty from the nobles, who hoped for positions with more privilege, but that was not true for the Kingdom of Poland, where the elites looked for ways to circumvent Alexis's succession.

[10] However, Denmark was not fighting on Polish soil, and although its involvement tied down Charles X Gustav's forces and a formal alliance with Poland was concluded in July, the Danish aimed to recover Scandinavian territories, which had been lost at the Second Treaty of Brömsebro (1645).

[11] Thus, the Habsburgs were interested in Frederick William I changing sides and sent diplomat Franz Paul Freiherr von Lisola to mediate a respective settlement.

[16][nb 1] The amended and final version of the treaty was ratified on 6 November by Frederick William I and John II Casimir in Bromberg (Bydgoszcz).

[17][nb 2] The Brandenburgian elector and the Polish king attended the ceremony with their wives, Luise Henriette of Nassau and Marie Louise Gonzaga, respectively.

[18] The Treaties of Wehlau and Bromberg were confirmed by the parties[19] and internationally recognized[20] at the Peace of Oliva, which ended the Second Northern War in 1660,[19] and by the Polish Sejm in 1659 and 1661.

[20] The Roman Catholic Church in the former Duchy of Prussia was to remain subordinate to the archbishop of Ermland (Warmia),[24] retain its possessions and income and be granted religious freedom.

[25][31] The Catholic communities were to stay subordinate to and to be represented by the Kuyavian bishop and keep all of their income, and the Electors of Brandenburg and the local nobility were to have patronage over the churches.

[32] The rights of the nobility of Lauenburg and Bütow Land were to be left unchanged, and previous court sentences and privileges were to remain in force.

[33] The ecclesiastical subordination of the Roman Catholic Church to the Polish Royal Prussian prince-bishop of Ermland (Warmia) also caused tensions with the House of Hohenzollern.

[15] That made Frederick William I not support Poland in the contemporary Russo-Polish War,[37] but he yielded the neutrality agreement that he had concluded with Russia in 1656.

[39] When Augustus the Strong failed to pay, the town was reoccupied in 1703, during the Great Northern War, but the Prussians again withdrew shortly afterwards because of Swedish pressure.

[42] In Article V of the Warsaw Treaty, Poland renounced its right to buy Draheim back and ceded it to Prussia "for eternal times".

Frost the House of Hohenzollern, Wehlau-Bromberg was a "major geopolitical gain and surge in wealth and prestige",[12][43] and Poland had "substantially benefited" from Brandenburgian support during the war.

[15] Christopher M. Clark says that John Casimir of Poland was "eager to separate Brandenburg from Sweden and to neutralize it as a military threat" when Poland–Lithuania was threatened by the Tsardom of Russia and ready to accept the Hohenzollerns' demands because of pressure by the House of Habsburg, which, after the emperor's incidental death earlier that year, needed to secure the elector's vote since its "urgings [...] carried considerable weight since the Poles were counting on Austrian assistance in the event of a renewed Swedish or Russian attack".

[47] Frost says that the treaty was subject to criticism of historians such as Kazimierz Piwarski, who considered that the price paid by Poland in Bromberg was unnecessarily high.