The period 1100-1500 was an age of increasing global contact through war and trade, dominated by the Mongol explosion of the thirteenth century. But it opened with the meeting of three civilizations and their military systems at the eastern end of the Mediterranean. The Crusades brought western Europe, Byzantium, and the Muslim world, with its connections to Central Asia’s nomads, into an extended period of competition and exchange. As holy wars, the Crusades represented the culmination of trends connecting war and religion; as expressions of civilizational expansion, they pointed to a growing connection between war, commerce, and cultural contact. The eventual fall of Byzantium showed that such contact could be disastrous, but all three civilizations also demonstrate the creative potential of contact.