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Claim to the caliphate and resumption of hostilities

Following the breakdown of the arbitration talks, Amr and the Syrian delegates returned to Damascus, where they greeted Mu'awiya as amir al-mu'minin, signaling their recognition of him as caliph. In April or May 658, Mu'awiya received a general pledge of allegiance from the Syrians. In response, Ali broke off communications with Mu'awiya, mobilized for war and invoked a curse against Mu'awiya and his close retinue as a ritual in the morning prayers. Mu'awiya reciprocated in kind against Ali and his closest supporters in his own domain.

In July, Mu'awiya dispatched an army under Amr to Egypt after a request for intervention from pro-Uthman mutineers in the province who were being suppressed by the governor, Caliph Abu Bakr's son and Ali's stepson, Muhammad. The latter's troops were defeated by Amr's forces, the provincial capital Fustat was captured and Muhammad was executed on the orders of Mu'awiya ibn Hudayj, leader of the pro-Uthman rebels. The loss of Egypt was a major blow to the authority of Ali, who was bogged down battling Kharijite defectors in Iraq and whose grip in Basra and Iraq's eastern and southern dependencies was eroding. Though his hand was strengthened, Mu'awiya refrained from launching a direct assault against Ali. Instead, his strategy was to bribe the tribal chieftains in Ali's army to his side and harry the inhabitants along Iraq's western frontier. The first raid was conducted by al-Dahhak ibn Qays al-Fihri against nomads and Muslim pilgrims in the desert west of Kufa. This was followed by Nu'man ibn Bashir al-Ansari 's abortive attack on Ayn al-Tamr then, in the summer of 660, Sufyan ibn Awf 's successful raids against Hit and Anbar.

In 659 or 660, Mu'awiya expanded the operations to the Hejaz (western Arabia, where Mecca and Medina are located), sending Abd Allah ibn Mas'ada al-Fazari to collect the alms tax and oaths of allegiance to Mu'awiya from the inhabitants of the Tayma oasis. This initial foray was defeated by the Kufans, while an attempt to extract oaths of allegiance from the Quraysh of Mecca in April 660 also failed.

In the summer, Mu'awiya dispatched a large army under Busr ibn Abi Artat to conquer the Hejaz and Yemen. He directed Busr to intimidate Medina's inhabitants without harming them, spare the Meccans and kill anyone in Yemen who refused to pledge their allegiance. Busr advanced through Medina, Mecca and Ta'if, encountering no resistance and gaining those cities' recognition of Mu'awiya. In Yemen, Busr executed several notables in Najran and its vicinity on account of past criticism of Uthman or ties to Ali, massacred numerous tribesmen of the Hamdan and townspeople from Sana'a and Ma'rib. Before he could continue his campaign in Hadhramawt, he withdrew upon the approach of a Kufan relief force. News of Busr's actions in Arabia spurred Ali's troops to rally behind his planned campaign against Mu'awiya, but the expedition was aborted as a result of Ali's assassination by a Kharijite in January 661.