Treaties of Wehlau and Bromberg

Course of events

The agreements with Elector Frederick William ended his alliance with Sweden. The Commonwealth recognized his sovereignty in Ducal Prussia.¹

A closer look

The Wehlau agreement was signed on 19 September and the Bromberg agreement on 5 November; the next day John Casimir and Frederick William swore to its terms outside the Jesuit church in Bydgoszcz. The elector left Sweden’s side but gained sovereignty in Ducal Prussia, previously a Crown fief. The later Peace of Oliva confirmed this settlement.¹

Why Ducal Prussia mattered

Since 1525 its ruler had been a vassal of the Polish king. During the Deluge Frederick William aligned with Sweden: he accepted Swedish protection in January 1656, then fought the Commonwealth and joined the Radnot plan. Military pressure and diplomacy led John II Casimir to make sweeping concessions in 1657 to win him over.¹

Two agreements and an oath

A preliminary agreement was signed at Wehlau on 19 September, a final one at Bydgoszcz on 5 November, and the two rulers swore to its terms before the Jesuit church the following day. Frederick William and his successors gained sovereignty in Ducal Prussia, subject to reversion to the Commonwealth if the Hohenzollern line died out.¹

The lesser known additions

Frederick William also received Drahim as a pledge and the Lębork-Bytów lands as a fief. In return he broke with Sweden. Parliament ratified the agreement in 1658, and the Peace of Oliva confirmed it in 1660. The loss of feudal overlordship over Ducal Prussia was a lasting consequence of the war.¹