Khalid ibn al-Walid
Elimination of Musaylima and conquest of the Yamama

Following a series of setbacks in her conflict with rival Tamim factions, Sajah joined the strongest opponent of the Muslims: Musaylima, the leader of the sedentary Banu Hanifa tribe in the Yamama, the agricultural eastern borderlands of Najd. Musaylima had laid claims to prophet-hood before Muhammad's emigration from Mecca, and his entreaties for Muhammad to mutually recognize his divine revelation were rejected by Muhammad. After Muhammad died, support for Musaylima surged in the Yamama, whose strategic value lay not only with its abundance of wheat fields and date palms, but also its location connecting Medina to the regions of Bahrayn and Oman in eastern Arabia. Abu Bakr had dispatched Shurahbil ibn Hasana and Khalid's cousin Ikrima with an army to reinforce the Muslim governor in the Yamama, Musaylima's tribal kinsman Thumama ibn Uthal. According to the modern historian Meir Jacob Kister, it was likely the threat posed by this army which compelled Musaylima to forge an alliance with Sajah. Ikrima was repelled by Musaylima's forces and thereafter instructed by Abu Bakr to quell rebellions in Oman and Mahra (central southern Arabia) while Shurahbil was to remain in the Yamama in expectation of Khalid's large army.

After his victories against the Bedouin of Najd, Khalid headed to the Yamama with warnings of the Hanifa's military prowess and instructions by Abu Bakr to act severely toward the tribe should he be victorious. The 12th-century historian Ibn Hubaysh al-Asadi holds that the armies of Khalid and Musaylima respectively stood at 4,500 and 4,000. Kister dismisses the much larger figures cited by most of the early Muslim sources as exaggerations. Khalid's first three assaults against Musaylima at the plain of Aqraba were beaten back. The strength of Musaylima's warriors, the superiority of their swords and the fickleness of the Bedouin contingents in Khalid's ranks were all reasons cited by the Muslims for their initial failures. Khalid heeded the counsel of the Ansarite Thabit ibn Qays to exclude the Bedouins from the next fight.

In the fourth assault against the Hanifa, the Muhajirun under Khalid and the Ansar under Thabit killed a lieutenant of Musaylima, who subsequently fled with part of his army. The Muslims pursued the Hanifa to a large enclosed garden which Musaylima used to stage a last stand against the Muslims. The enclosure was stormed by the Muslims, Musaylima was slain and most of the Hanifites were killed or wounded. The enclosure became known as the 'garden of death' for the high casualties suffered by both sides.

Khalid assigned a Hanifite taken captive early in the campaign, Mujja'a ibn al-Murara, to assess the strength, morale and intentions of the Hanifa in their Yamama fortresses in the aftermath of Musaylima's slaying. Mujja'a had the women and children of the tribe dress and pose as men at the openings of the forts in a ruse to boost their leverage with Khalid; he relayed to Khalid that the Hanifa still counted numerous warriors determined to continue the fight against the Muslims. This assessment, along with the exhaustion of his own troops, compelled Khalid to accept Mujja'a's counsel for a ceasefire with the Hanifa, despite Abu Bakr's directives to pursue retreating Hanifites and execute Hanifite prisoners of war.

Khalid's terms with the Hanifa entailed the tribe's conversion to Islam and the surrender of their arms and armor and stockpiles of gold and silver. Abu Bakr ratified the treaty, though he remained opposed to Khalid's concessions and warned that the Hanifa would remain eternally faithful to Musaylima. The treaty was further consecrated by Khalid's marriage to Mujja'a's daughter. According to Lecker, Mujja'a's ruse may have been invented by the Islamic tradition "in order to protect Khalid's policy because the negotiated treaty... caused the Muslims great losses". Khalid was allotted an orchard and a field in each village included in the treaty with the Hanifa, while the villages excluded from the treaty were subject to punitive measures. Among these villages were Musaylima's hometown al-Haddar and Mar'at, whose inhabitants were expelled or enslaved and the villages resettled with tribesmen from clans of the Tamim.
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