At its zenith, the Mongol Empire was the largest the world has ever seen. The Mongols dominated China, the Middle East, large portions of Russia, and most of Eastern Europe. An attempt to expand the empire into Japan failed when the largest fleet assembled before World War II was destroyed by a typhoon. The Japanese denominated the typhoon as a "divine wind," or kamakaze, a term that became synonymous with sacrifice in World War II. Their large Empire actually encouraged rather than inhibited inter-cultural exchange. The Silk Roads, the primary means of exchange between Europe and Asia were heavily policed by the Mongols which allowed trade and ideas to pass freely without fear of Bandits. The Mongols brought in outsiders to administer their territories locally. They were not administrators, and did not trust the local people, so others, even from Europe, were brought in. A classic example of this is Marco Polo who presumably was made governor of a Chinese province by Kublai Khan. Genghis Khan's favorite wife was a Christian, so he also was curious to learn more about this religion. Mongol exchange of ideas did not extend to political alliances, however. Once when the Pope proposed an alliance to defeat the Muslims, the Mongol Khan declined, and advised the Pope to submit to Mongol rule or be destroyed.