the oldest Serbian ruler on which there is substantial data. Vlastimir, Mutimir and Pribislav Vlastimir united the Serbian tribes in the vicinity. The Serbs were alarmed, and most likely consolidated due to the spreading of the Bulgarian Khanate towards their borders (a rapid conquest of neighbouring Slavs,) in self-defence, and possibly sought to cut off the Bulgar expansion to the south. Byzantine Emperor Theophilos (r. 829–842) was recognized as the nominal suzerain (overlord) of the Serbs, and most likely encouraged them to thwart the Bulgarians. The thirty-year-peace treaty between the Byzantines and Bulgarians, signed in 815, was still in effect. According to Constantine VII, the Serbs and Bulgarians had lived peacefully as neighbours until the invasion in 839 (in the last years of Theophilos). It is not known what exactly prompted the war, as Porphyrogenitus gives no clear answer; whether it was a result of Serbian-Bulgarian relations, i.e., the Bulgar conquest to the southeast, or a result of the Byzantine-Bulgarian rivalry, in which Serbia was allied with the Byzantines. According to John B. Bury, it was not unlikely that the Emperor had a part in it; as he was at war with the Arabs, he may have pushed the Serbs to drive the Bulgarians from western Macedonia, which would benefit them both—hence, Malamir's action. V. Zlatarski supposed that the Emperor offered the Serbs complete independence in return. According to Porphyrogenitus, the Bulgarians wanted to continue their conquest of the Slav lands - to force the Serbs into subjugation. Presian I (r. 836–852) launched an invasion into Serbian territory in 839, which led to a war that lasted for three years, in which the Serbs were victorious; Presian was heavily defeated and lost a large number of his men, made no territorial gains and was driven out by the army of Vlastimir. It is believed that the Serbs held out in their defensible forests and gorges, and knew how to fight in the hills. According to Živković, it is possible that the Bulgarian attack came after the failed invasion of Struma and Nestos in 846 (see below): Presian may have collected his army and headed for Serbia, and Vlastimir may have participated in the Byzantine–Bulgarian Wars, which would mean that Presian was responding to a direct Serbian involvement. After the victory over the Bulgarians, Vlastimir's status rose, and according to Fine, he went on to expand to the west, taking Bosnia