[1] Abu Ubayda's father Abd Allah was among the chiefs of the Quraysh in the Fijar War against the Hawazin nomads in the late 6th century.

He was also made commander of a small expedition that set out to attack and destroy the tribes of Tha'libah and Anmar, who were plundering nearby villages.

The other faction of the Muhajirun, mostly backed Abu Bakr due to his seniority, closeness to Muhammad, and the increasingly important role he had attained in the prophet's last years.

[10] A report in the history of al-Baladhuri holds that after Muhammad died, Umar told Abu Ubayda, "Stretch your hand and let us give you the bay'ah [oath of allegiance], for you are the custodian (Amīn) of this ummah (the Muslim community), as the prophet called you".

Several accounts in the Islamic tradition claim Abu Ubayda concealed the caliph's order from the rest of the army to avoid potentially insulting Khalid or provoking a mutiny while the Muslims were on the cusp of a major confrontation with the Byzantines.

[14] Athamina dismisses the reliability of these claims, considering them militarily illogical and meant to dramatise the change in command and emphasise Abu Ubayda's "moral superiority and unselfishness".

[15] Abu Ubayda may have been chosen to lead at that time, when the Byzantine defence of Syria had taken an enormous blow, as the circumstances called for an able administrator to take the helm from a military commander like Khalid.

[16] The Islamic tradition provides a host of moral and personal reasons why Khalid was demoted in favour of Abu Ubayda, but most modern historians view these as either partially valid or literary innovations.

Athamina holds Abu Ubayda was likely installed because Khalid and his large force of tribesmen from Arabia and Iraq, along with their families, presented a threat to the old-established, formerly Byzantine-allied, and militarily experienced Arab tribes of Syria, whose defection was considered vital by Umar to form a network of defences against the Byzantines.

Caloiis and Azrail, the governor of Damascus, led another army to stop Khalid's corps but they were also defeated in the battle of Maraj-al-Safar on 19 August 634.

After defeating the Byzantine reinforcements sent by Emperor Heraclius at the Battle of Sanita-al-Uqab, 20 mi (32 km) from Damascus, Khalid's forces attacked and entered the city.

After the three-day truce was over, the Muslim cavalry, under Khalid's command, pursued the Byzantine column via the shorter Emessa road and caught up with them in the northwest Beqaa Valley, just before they entered the mountains en route to Antioch at the Battle of Maraj-al-Debaj.

[18] Soon after the appointment of Abu-Ubayda as commander in chief, he sent a small detachment to the annual fair held at Abu-al-Quds, modern day Abla, near Zahlé; east of Beirut.

Khalid reached there in time and defeated the garrison in the Battle of Abu-al-Quds on 15 October 634 and returned with tons of looted booty from the fair and hundreds of Byzantine prisoners.

Abu Ubayda decided to march to Fahl (Pella), which is about 500 ft (150 m) below sea level, and where a strong Byzantine garrison and survivors of the Battle of Ajnadayn were present.

So rather than invading the districts of Emesa and Chalcis, he consolidated his rule in conquered land and captured Hama, Maarrat al-Nu'man.

Having mustered sizeable armies at Antioch, Heraclius sent them to reinforce strategically important areas of northern Syria, such as Emesa and Chalcis.

In a council of war he suggested that Abu Ubayda draw all the Muslim armies to one place so as to fight a decisive battle with the Byzantines.

Agreeing with Khalid's suggestion, Abu Ubayda ordered all the Muslim armies in Syria to evacuate the lands they had conquered and meet at Jabiya.

This manoeuvre turned out to be a decisive blow to the Heraclius's plan, as he did not wish engage his troops in an open battle with the Muslims, where their light cavalry could be effectively used.

On Khalid's suggestion, Abu Ubayda ordered the Muslim army to withdraw from Jabiya to the plain on the Yarmouk River, where cavalry could be used.

The Byzantine commander in chief, Vahan, sent Christian Arab troops of the Ghassanid king, Jabalah ibn al-Aiham, to check the strength of the Muslims.

[21] Khalid was in favour of an open battle outside the fort, but Abu Ubayda sought Umar's advice, who handled the situation brilliantly.

Menas, diverting from conventional Byzantine tactics, decided to face Khalid and destroy the leading elements of the Muslim army before the main body could join them at Hazir, 5 km (3.1 mi) east of Chalcis.

Khalid and Abu Ubayda continued their march northward and laid siege to Aleppo, which was captured after fierce resistance from desperate Byzantine troops in October 637.

Accordingly, they sent detachments north to eliminate all possible Byzantine forces and captured the garrison town of Azaz, 50 km (31 mi) from Aleppo.

Abu Ubayda sent Khalid northwards, while he marched south and captured Lazkia, Jabla, Tartus and the coastal areas west of Anti-Lebanon Mountains.

They marched independently and captured Edessa, Amida (Diyarbakır), Malatya and whole of Armenia up to Ararat and raided northern and central Anatolia.

Heraclius had already abandoned all the forts between Antioch and Tartus to create a buffer zone or no man's land between Muslim controlled areas and mainland Anatolia.

[25] The Jarrahs, the family that traditionally maintained the tomb, claimed descent from Abu Ubayda, and were exempt by the Ottoman authorities from paying taxes.