The Expedition of Usama bin Zayd also known as the Army of Usama ibn Zaid to al-Balqa, took place in 11AH of the Islamic calendar, in May 632. Usama ibn Zayd was appointed as the commander of an expeditionary force which was to invade Palestine again (and attack Moab and Darum), on the orders of Muhammad. He set out with 3000 men on the same day Muhammad died, when he reached the destination he attacked the inhabitants, killing many, taking as many captives as he could.
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Preparation
Muhammad invited Usama ibn Zayd (son of Zayd ibn Harithah) to a mosque and ordered him to act as the commander of an army that was to invade Palestine, and attack Takhum of al-Balqa (which was in Palestine). Usama ibn Zayd was the son of Zayd ibn Harithah, a slave that was very close to Muhammad whom he had freed. Zayd ibn Haritha was killed in the Battle of Mutah.
In addition to attacking Balqa, he was ordered to attack Darum. Some weeks later, Muhammad fell ill, and from his seat (the Minbar) in the mosque, he ordered that Usama ibn Zayd should lead the expeditionary force. Muhammad also rebuked those that claimed he did not merit such an honour, or that he was too young, while the best of Muhammad’s commanders were available.
Usama visited Muhammad before he went into battle. The next day he set out for his expedition and learnt Muhammad had died on 8 June 632. He was told by Abu Bakr to continue the expedition.
10 Rules of Abu Bakr
According to Tabari, before Usamah headed out, Abu Bakr advised Usamah with "10 things", which were like his rules of war. The tradition about the 10 "things" of Abu Bakr are also mentioned in the Sunni Hadith collection Al-Muwatta. The tradition mentioned many things, including leaving "monks" alone. Imam Shaffi (founder of the Shaffi school of thought) did not consider the tradition, about the 10 rules of Abu Bakr as authentic, but the same book which claimed he did not consider it authentic, also mentioned in another occasion that Shaffi considered it authentic (or partly authentic), and used it to justify killing monks only if they fought Muslims. But the same book explains that even if it was authentic, it does not mean that monks can not be killed, and claims that Abu Bakr's intention (according to Shaffi) was to only temporarily leave the monasteries alone. Shaffi concluded that "monks" are not included in his list of "non combatants". Abu Yusuf mentioned a counter tradition about the instructions of Abu Bakr, which claimed that Abu Bakr ordered his commanders to lay waste to every village where he did not hear the call to prayer.
Invasion
Usama headed out with 3000 men, of which 1000 were cavalry soldiers. Abu Bakr accompanied Usama part of the way. Usama had also sent a spy, from which he learned that the inhabitants were still unaware of the imminent approach of the army.