The fact is that the Mongol empire, which had functioned very effectively across such a huge expanse for the first half of the thirteenth century proved unable to hold together when its guiding principle—unwavering loyalty to the authority of a single, undisputed leader—was challenged. The extraordinary postal communications system established under Ögödei, which allowed commanders to keep in contact with one another while fighting thousands of miles apart, was no use if those commanders decided that they were more interested in their own gain than that of the supreme khan
