Why did the Order of the Brothers of the Sword become part of the Teutonic Order?

The Order of the Brothers of the Sword was forced to disband and reorganize as a branch of the Teutonic Order in 1237 following a catastrophic military collapse.12

In September 1236, the order suffered a disastrous defeat at the Battle of Saule against a combined force of Semigallians and Samogitians, which resulted in the death of their grand master, Volquin, and the effective destruction of their military power.13

Prior to this defeat, the order had faced significant criticism from both the pope and the Holy Roman emperor for its brutal tactics against both heathens and recent converts. Furthermore, the order appeared increasingly focused on establishing its own feudal domain rather than fulfilling its original mission of gathering converts for the church. Consequently, the pope compelled the remnants of the order to merge with the Teutonic Knights, whose grand master thereafter appointed the provincial master (Landmeister) of the newly formed Livonian branch.134