The first Prussian uprising was influenced by three major events. Firstly, the Livonian Knights – a subsidiary of the Teutonic Knights – lost the Battle of the Ice on Lake Peipus to Alexander Nevsky in April 1242. Secondly, southern Poland was devastated by a Mongol invasion in 1241; Poland lost the Battle of Legnica and the Teutonic Knights lost one of its most trusted allies that often supplied troops. Thirdly, Duke Swantopolk II of Pomerania was fighting against the Knights, who supported his brothers' dynastic claims against him. It has been implied that the new castles of the Knights were competing with his lands over the trade routes along the Vistula River. While some historians embrace the Swantopolk–Prussian alliance without hesitation, others are more careful. They point out that the historical information came from documents written by the Teutonic Knights and must have been ideologically charged to persuade the Pope to declare a crusade not only against the pagan Prussians but also against the Christian duke.