In the loth century we have again the means of observing how the Government conducted its foreign policy on carefully thought out principles. The empire was then exposed to constant danger from Bulgaria, to inroads of the Magyars, and to attacks of the Russians. The key to the diplomatic system, designed to meet these dangers, was the cultivation of friendly relations with the Petchenegs, who did not menace the provinces either by land or sea and could be incited to act against Russians, Bulgarians or Magyars. The system is explained in the treatise (known as De administrando imperio) composed by the emperor Constantine Porphyrogennetos (c. 95o). The series of these northern States was completed by the kingdom of the Khazars (between the Cau casus and the Don), with which the empire had been in relation since the time of Heraclius, who, to win its co-operation against Persia, promised his daughter in marriage to the king. Afterwards the Khazars gave two empresses to New Rome (the wives of Jus tinian II. and Constantine V.). Their almost civilized State steered skilfully between the contending influences of Islam and Chris tianity, and its kings adopted the curious means of avoiding sus picion of partiality for either creed by embracing the neutral reli gion of the Jews. Commercial and political relations with the Kha zars were maintained through the important outpost of the empire at Cherson in the Crimea, which had been allowed to retain its re publican constitution under a president (rpcorebwv) and a munic ipal board ( etpxovres ), though this freedom was limited by the appointment of a strategos in 833, a moment at which the Khazars were seriously threatened by the Petchenegs. The danger to be feared from the Khazars was an attack upon Cherson, and it seems probable that this was a leading consideration with Leo III. when
he wedded his son Constantine V. to a Khazar princess. In the 9th century it was an object of the Government to maintain the Kha zars (whose army consisted mainly of mercenaries) against the Petchenegs; and hence, if it should become necessary to hold the Khazars in check, the principle was to incite against them not the Petchenegs, but other less powerful neighbours, the Alans of the Caucasus, and the people of "Black Bulgaria" on the middle Volga (a State which survived till the Mongol conquest).
For this systematic diplomacy it was necessary to collect infor mation about the peoples whom it concerned. The ambassadors sent to the homes of barbarous peoples reported everything of interest they could discover. We owe to Priscus a famous graphic account of the embassy which he accompanied to the court of Attila. We possess an account of an embassy sent to the Turks in central Asia in the second half of the 6th century, derived from an official report. Peter the Patrician in Justinian's reign drew up careful reports of his embassies to the Persian court. When for eign envoys came to Constantinople, information was elicited from them as to the history and domestic politics of their own countries. It can be shown that some of the accounts of the history and cus toms of neighbouring peoples, stored in the treatise of Constantine Porphyrogennetos referred to above (furnishing numerous facts not to be found anywhere else), were derived from barbarian ambassadors who visited Constantinople, and recorded by the imperial secretaries. We may conjecture with some probability that the famous system of the Relazioni, which the Venetian Gov ernment required from its ambassadors, goes back originally to Byzantine influence.