the ugly byzantine
Byzantine diplomacy was very expensive. Dowries, gifts, subsidies to whole nations, all involved the treasury in enormous sums. Even economic blockades, sometimes effectively employed towards the Saracens, were costly for the Empire also. The Government was moreover perfectly willing to pay its enemies direct not to invade its territory. Lawless princes across the frontier thus became clients, almost wage-earners, much preferring a regular income of Byzantine gold to the uncertain takings of a raid. At times even, if Byzantium was for some reason unwilling to undertake a war, a yearly some of money would go to Baghdad or Preslav. The Calif or the Tsar might call it a tribute, if he chose. To the Emperor it was merely a wise investment; when he was ready to fight the payment would cease. But it all depended on a full treasury. So long as the money was there Byzantine diplomacy flourished. But when Constantinople was no longer the financial centre of the world then there came the decline.
(Byzantine Civilization, p. 129)