At the commencement of his reign, Murad turned to conquest. The work of Orchan had been to establish and compact Ottoman rule in Asia Minor. That of his successor was mainly to carry out a similar policy in Europe. After capturing Heraclia on the Black Sea, he crossed over into Thrace and occupied Adrianople, seized Didymotica and Chorlou, overran the whole country between Constantinople and Bulgaria, and sent his ships to plunder the Greek islands. In return for the fanaticism with which they had inspired him, he promised that one fifth of the spoil captured by land and sea should be given to the mollahs. When the sale of Christian captives took place, he took care, says Ducas,[90] that the young, the well set-up, and the strong men should be bought at a low price to be added to the Janissaries. The few remaining Turkish emirs in Asia Minor whose territories had not been gained by the Ottomans joined forces to resist the new sultan. At the same time the Serbians, Bulgarians, and Hungarians, all of whom had become alarmed at Murad’s progress, declared war upon him. Compelled in 1363 to defend himself against the emirs to the east and south of his territories in Asia Minor, he was sufficiently strong to force the emperor to bind himself not merely to give aid to him in Asia but not to attempt to recover any of the cities or territories which he had conquered in Europe. When he had broken the strength of the rebel emirs he crossed rapidly back into Thrace and near Adrianople defeated a combined army of Hungarians, Serbians, and Bulgarians. Two years afterwards, in 1366, an army of fifty thousand Serbians endeavoured in vain to drive Murad out of Adrianople. The lowest degradation which the empire had yet reached was when the miserable John consented to become the tributary of Murad in order that he might enjoy his remaining possessions in Europe. In 1373 he formally recognised the sultan as his suzerain, bound himself to render him military service and to give his son Manuel as a hostage.[91]