The Bulgars taking command over the Slav population in Bulgaria began a warfare against the enfeebled Greek Empire. That Empire gave up Moesia to the Bulgarian King, Isperich, and agreed to pay him a tribute, it being the custom of the degenerate descendants of the Roman Empire of the period thus to attempt to buy safety with bribes. The Emperor Justinian II. stopped this tribute, and a war followed, in which the Bulgarians were successful, and Justinian lost his throne and was driven to exile. Later, Justinian made another treaty with the Bulgarians and offered his daughter in marriage to the new Bulgarian King, Tervel, and with Bulgarian help he was restored to his throne. But war between the Bulgars and the Empire was chronic. To quote a Bulgarian chronicler: The chief characteristics of the Bulgars were warlike virtues, discipline, patriotism, and enthusiasm. The Bulgarian kings brought their victorious armies to the gates of Constantinople, whose very existence they threatened. The Greek Emperor sought their friendship, and even consented to pay them tribute. Bulgaria attained her greatest empire in the reign of King Kroum. Between King Isperich and King Kroum, however, Bulgaria had many ups and downs. The Bulgarian King, Kormisos, once almost reached the walls of Constantinople. But trouble among his own people prevented his victories being pushed home. Then a series of civil wars in Bulgaria weakened the nation, and a great section of it migrated to Asia Minor. The Roman Emperor, Constantine V., took this occasion to exact a full revenge for previous Bulgar attacks on Constantinople. The Bulgar army was routed, and an invading force carried the torch into every Bulgarian town. A new Bulgar King, Cerig, restored his country's position somewhat by a secretly plotted massacre of all its enemies within its boundaries. The Empress Irene then ascended the Imperial throne at Constantinople and found herself unable to withstand the Bulgar power, and went back to the system of paying tribute to the Bulgarians as the price of safety. King Kroum next ascended the throne of Bulgaria and, capable and savage warrior as he was, raised its power vastly. He defeated and slew the Greek Emperor, Nicephorus, in battle, and captured Sofia (809), the present capital of Bulgaria. Warfare was savage in those days, and between the Bulgars and the Greek emperors particularly savage. The defeated Imperial army was massacred to a man, from the Emperor down to the foot-soldier. King Kroum afterwards used the skull of the descendant of the Caesars as a drinking-cup.