The widow of late King Zvonimir, Helen, tried to keep her power in Croatia during the succession crisis. Some Croatian nobles around Helen, possibly the Gusić family and/or Viniha from Lapčan family, contesting the succession after the death of Zvonimir, asked King Ladislaus I to help Helen and offered him the Croatian throne, which was seen as rightfully his by inheritance rights. According to some sources, several Dalmatian cities also asked King Ladislaus for assistance, and Petar Gusić with Petar "de genere Cacautonem" presented themselves as "White Croats" ("Creates Albi"), on his court. Thus the campaign launched by Ladislaus was not purely a foreign aggression nor did he appear on the Croatian throne as a conqueror, but rather as a successor by hereditary rights. In 1091 Ladislaus crossed the Drava river and conquered the entire province of Slavonia without encountering opposition, but his campaign was halted near the "Forest Mountain" (Mount Gvozd). Since the Croatian nobles were divided, Ladislaus had success in his campaign, yet he wasn't able to establish his control over the entirety of Croatia, although the exact extent of his conquest is not known. At this time the Kingdom of Hungary was attacked by the Cumans, who were likely sent by Byzantium, so Ladislaus was forced to retreat from his campaign in Croatia. Ladislaus appointed his nephew Prince Álmos to administer the controlled area of Croatia, established the Diocese of Zagreb as a symbol of his new authority and went back to Hungary. In the midst of the war, Petar Svačić was elected king by Croatian feudal lords in 1093. Petar's seat of power was based in Knin. His rule was marked by a struggle for control of the country with Álmos, who wasn't able to establish his rule and was forced to withdraw to Hungary in 1095.