The most impactful religious development occurred when the Kipchak Khanate or Golden Horde supported Islam as the official state religion. Unlike the religious tolerance of previous years, the Golden Horde's strong stance on religion led the Empire to begin an assault on the trading post of Kaffa in 1346 in an attempt to end trade with Christians. During their attack on Kaffa, the Mongol soldiers contracted the Black Death and were forced to withdraw their military forces. Innovatively, the Mongols exploited the weakness of the dispersed soldiers to their advantage by participating in a very early form of biological warfare. Diseased corpses were catapulted into the city of Kaffa in hopes of infecting its citizens. The diseased corpses killed the citizens of Kaffa and while merchant ships continued to enter Kaffa harbor trading goods, the ships continued to trade throughout the Mediterranean and spread the Black Death throughout Europe. Clearly, the Mongol Empire was partly responsible for the deaths of what by some estimates was 60% of the European population due to blacks.