formed alliances with and against one another. The Pechenegs were nomads roaming the steppe raising livestock which they traded with the Rus' for agricultural goods and other products. The lucrative Rus' trade with the Byzantine Empire had to pass through Pecheneg-controlled territory, so the need for generally peaceful relations was essential. Nevertheless, while the Primary Chronicle reports the Pechenegs entering Rus' territory in 915 and then making peace, they were waging war with one another again in 920. Pechenegs are reported assisting the Rus' in later campaigns against the Byzantines, yet allied with the Byzantines against the Rus' at other times. Rus'–Byzantine relations After the Rus' attack on Constantinople in 860, the Byzantine Patriarch Photius sent missionaries north to convert the Rus' and the Slavs to Christianity. Prince Rastislav of Moravia had requested the Emperor to provide teachers to interpret the holy scriptures, so in 863 the brothers Cyril and Methodius were sent as missionaries, due to their knowledge of the Slavonic language. The Slavs had no written language, so the brothers devised the Glagolitic alphabet, later replaced by Cyrillic (developed in the First Bulgarian Empire) and standardized the language of the Slavs, later known as Old Church Slavonic. They translated portions of the Bible and drafted the first Slavic civil code and other documents, and the language and texts spread throughout Slavic territories, including Kievan Rus'. The mission of Cyril and Methodius served both evangelical and diplomatic purposes, spreading Byzantine cultural influence in support of imperial foreign policy. In 867 the Patriarch announced that the Rus' had accepted a bishop, and in 874 he speaks of an "Archbishop of the Rus'." Relations between the Rus' and Byzantines became more complex after Oleg took control over Kiev, reflecting commercial, cultural, and military concerns. The wealth and income of the Rus' depended heavily upon trade with Byzantium. Constantine Porphyrogenitus described the annual course of the princes of Kiev, collecting tribute from client tribes, assembling the product into a flotilla of hundreds of boats, conducting them down the Dnieper to the Black Sea, and sailing to the estuary of the Dniester, the Danube delta, and on to Constantinople. On their return trip they would carry silk fabrics, spices, wine, and fruit. The importance of this trade relationship led to military action when disputes arose. The Primary Chronicle reports that the Rus' attacked Constantinople again in 907, probably to secure trade access. The