By the 880s, Khazar control of the Middle Dnieper from Kyiv, where they collected tribute from Eastern Slavic tribes, began to wane as Oleg of Novgorod wrested control of the city from the Varangian warlords Askold and Dir, and embarked on what was to prove to be the foundation of a Rus' empire. The Khazars had initially allowed the Rus' to use the trade route along the Volga River, and raid southwards. See Caspian expeditions of the Rus'. According to Al-Mas'udi, the qağan is said to have given his assent on the condition that the Rus' give him half of the booty. In 913, however, two years after Byzantium concluded a peace treaty with the Rus' in 911, a Varangian foray, with Khazar connivance, through Arab lands led to a request to the Khazar throne by the Khwârazmian Islamic guard for permission to retaliate against the large Rus' contingent on its return. The purpose was to revenge the violence the Rus' razzias had inflicted on their fellow Muslim believers. The Rus' force was thoroughly routed and massacred. The Khazar rulers closed the passage down the Volga to the Rus', sparking a war. In the early 960s, Khazar ruler Joseph wrote to Hasdai ibn Shaprut about the deterioration of Khazar relations with the Rus': "I protect the mouth of the river (Itil-Volga) and prevent the Rus arriving in their ships from setting off by sea against the Ishmaelites and (equally) all (their) enemies from setting off by land to Bab."