The second uprising, known in historiography as "the great Prussian uprising", was prompted by the 1260 Battle of Durbe, the largest defeat suffered by the Teutonic Knights in the 13th century. This uprising was the longest, largest, and most threatening to the Teutonic Order, who again were reduced to five of their strongest castles. Reinforcements for the Knights were slow to arrive, despite repeated encouragements from Pope Urban IV, and the position of the Order looked set to worsen. Luckily for the Order, the Prussians lacked unity and a common strategy and reinforcements finally reached Prussia in around 1265. One by one, Prussian clans surrendered and the uprising was ended in 1274.