Časlav recognized the supreme authority of the Byzantine emperor Constantine VII Porphyrogenitus, who became his mentor and protector. The emperor helped Časlav to restore Serbia, including the heavy financial aid. Časlav repopulated Serbia returning some of the people who fled to the neighboring countries. Serbia prospered, keeping good relations with the Byzantines and the emperor constantly pointed out the good relations with Serbia in this period. The loose demarcation of the boundaries in this period is the Croatian border on the west, with majority of Bosnia within the Časlav's state, the town of Ras on the east and the Sava river on the north. As he is the last archon mentioned by Constantine, it is believed that he was still alive and in power when the chapters on Serbs were written, in 949 or 950.