The invading Mongols accelerated the fragmentation of the Rus'. In 1223, the disunited southern princes faced a Mongol raiding party at the Kalka River and were soundly defeated. In 1237–1238 the Mongols burnt down the city of Vladimir (4 February 1238) and other major cities of northeast Russia, routed the Russians at the Sit' River, and then moved west into Poland and Hungary. By then they had conquered most of the Russian principalities. Only the Novgorod Republic escaped occupation and continued to flourish in the orbit of the Hanseatic League.