Patriarch Nicholas was presently forced to make peace with Tsar Simeon of Bulgaria, whom he reluctantly recognized as Bulgarian emperor. Because of this unpopular concession, Patriarch Nicholas was driven out of the regency by Constantine's mother Zoe. She was no more successful with the Bulgarians, who defeated her main supporter, the general Leo Phokas, in 917. In March 919, she was replaced as regent by the admiral Romanos Lekapenos, who married his daughter Helena Lekapene to Constantine. Romanos used his position to advance to the ranks of "basileopatōr" in May 919, to "kaisar" (Caesar) in September 920, and finally to co-emperor in December 920. Thus, just short of reaching nominal majority, Constantine was eclipsed by a senior emperor.