In the following years, attempts were made to return to Polish suzerainty, especially by the capital city of Königsberg, whose burghers rejected the treaties and viewed the region as part of Poland.

After losing a war against the Kingdom of Poland, and with his personal bishop, Georg von Polenz of Pomesania and of Samland, who had converted to Lutheranism in 1523,[6] and a number of his commanders already supporting Protestant ideas, Albert began to consider a radical solution.

At Wittenberg in 1522, and at Nuremberg in 1524, Martin Luther encouraged him to convert the order's territory into a secular principality under his personal rule, as the Teutonic Knights would not be able to survive the reformation.

[7] On 10 April 1525, Albert resigned his position, became a Protestant and in the Prussian Homage was granted the title "Duke of Prussia" by his uncle, King Sigismund I of Poland.

The knights who disapproved of the decision were pressured into acceptance by Albert's supporters and the burghers of Königsberg, and only Eric of Brunswick-Wolfenbüttel, Komtur of Memel, opposed the new duke.

Some of the knights converted to Lutheranism in order to retain their property and then married into the Prussian nobility, while others returned to the Holy Roman Empire, and remained Catholic.

On 1 March 1526, Albert married Princess Dorothea, daughter of King Frederick I of Denmark, thereby establishing political ties between Lutheranism and Scandinavia.

Albert was greatly aided by his elder brother George, Margrave of Brandenburg-Ansbach, who had earlier established the Protestant religion in his territories of Franconia and Upper Silesia.

The combination of taxation by the nobility, the contentions of the Protestant Reformation, and the abrupt secularization of the Teutonic Order's remaining Prussian lands exacerbated peasant unrest.

Although he was formally a vassal of the crown of Poland, Albert retained self-government for Prussia, his own army, the minting of his currency, a provincial assembly, (de, Landtag), and substantial autonomy in foreign affairs.

Joachim petitioned his brother-in-law, king Sigismund II of Poland the co-enfeoffment of his line of the Hohenzollern with the Prussian dukedom, and finally succeeded, including the then usual expenses.

The legal contradiction made a cross-border real union impossible; however, in practice, Brandenburg and Ducal Prussia were more and more ruled as one and were colloquially referred to as Brandenburg-Prussia.

His son, George William, was successfully invested with the duchy in 1623 by King of Poland Sigismund III Vasa, thus the personal union Brandenburg-Prussia was confirmed.

[10] Many of the Prussian Junkers were opposed to rule by the House of Hohenzollern of Berlin and appealed to Sigismund III Vasa for redress, or even incorporation of Ducal Prussia into the Polish kingdom, but without success.

Yet, during the Second Northern War, Charles X Gustav of Sweden invaded Ducal Prussia and dictated the Treaty of Königsberg (January 1656), which made the duchy a Swedish fief.

When the tide of the war turned against Charles X Gustav, he concluded the Treaty of Labiau (November 1656), making Frederick William I the full sovereign in Ducal Prussia and Warmia, which, however, was part of Poland.

The burghers of the capital city of Königsberg, led by Hieronymus Roth, rejected the treaties of Wehlau and Oliva and viewed Prussia as "indisputably contained within the territory of the Polish Crown".

In 1675, the Polish-French Treaty of Jaworów was signed according to which France was to support Polish efforts to regain control of the region, and Poland was to join the ongoing Franco-Brandenburgian War on the French side,[17] however, it was not implemented.