Khalid ibn walid's biography on Calif Abu bakr era.

Conquest of Arabia
Further information: Ridda wars and Malik ibn Nuwayrah

After the death of Muhammad, many powerful Arab tribes broke away in open revolt against the rule of Medina. Caliph Abu Bakr sent his armies to counter the rebels and apostates. Khalid was one of Abu Bakr’s main advisers and an architect of the strategic planning of the Riddah wars. He was given the command over the strongest Muslim army and was sent towards central Arabia, the most strategically sensitive area where the most powerful rebel tribes resided. The region was closest to the Muslim stronghold of Medina and was the greatest threat to the city. Khalid first set out for the rebel tribes of Tayy and Jalida, where Adi ibn Hatim—a prominent companion of Muhammad, and a chieftain of the Tayy tribe—arbitrated, and the tribes submitted to the Caliphate.

In mid-September 632 AD, Khalid defeated Tulaiha,[39] a main rebel leader who claimed prophethood as a means to draw support for himself. Tulaiha's power was crushed after his remaining followers were defeated at the Battle of Ghamra. Khalid next marched to Naqra and defeated the rebel tribe of Banu Saleem at the Battle of Naqra. The region was secured after the Battle of Zafar in October 632 with the defeat of a tribal mistress, Salma.

Once the region around Medina, the Islamic capital, was recaptured, Khalid entered Nejd, a stronghold of the Banu Tamim tribes. Many of the clans hastened to visit Khalid and submit to the rule of the Caliphate. But the Banu Yarbu' tribe, under Sheikh Malik ibn Nuwayrah, hung back. Malik avoided direct contact with Khalid's army and ordered his followers to scatter, and he and his family apparently moved away across the desert.[41] He also collected taxes and sent his men to Medina to deliver them. Nevertheless, Malik was accused of rebelling against the state of Medina and charged for entering into an anti-Caliphate alliance with Sajjah, a self-proclaimed prophetess.[42] Malik was arrested along with his clansmen,[43] and asked by Khalid about his crimes. Upon hearing Malik's response: "your master said this, your master said that" referring to Abu Bakr, Khalid declared Malik a rebel apostate and ordered his execution.

Abu Qatada Ansari, a companion of Muhammad, who accompanied Khalid from Medina was so shocked at Malik's murder by Khalid that he immediately returned to Medina, and told Abu Bakr that he refused to serve under a commander who had killed a Muslim. The death of Malik and Khalid's taking of his wife Layla created controversy. Some officers of his army—including Abu Qatadah—believed that Khalid killed Malik to take his wife. After the pressure exerted by Umar—Khalid's cousin and one of Caliph Abu Bakr's main advisors—Abu Bakr called Khalid back to Medina to explain himself. Although Khalid had declared Malik an apostate, in Medina, ‘Umar told Khâlid: “You enemy of Allâh! You killed a Muslim man and then leap upon his wife. By Allâh, I will stone you".Some have argued that Umar later dismissed him from army service over this.

Khalid then crushed the most powerful threat to the nascent Islamic state of Medina: Musaylimah, a claimant to prophethood, who had already defeated two Muslim armies. In the third week of December 632, Khalid won a decisive victory against Musaylimah at the Battle of Yamama. Musaylimah died in the battle, and nearly all resistance from rebelling tribes collapsed.

Invasion of Persian Empire

The route of Khalid ibn Walid's conquest of lower Mesopotamia (Iraq).
Further information: Islamic conquest of Persia
With the collapse of the rebellion, and Arabia united under the central authority of the caliph at Medina, Abu Bakr decided to expand his empire. It is unclear what his intentions were, whether it was a full scale expansion plan or pre-emptive attacks to secure more territory to create a buffer zone between the Islamic state and the powerful Sassanid and Byzantine empires. Khalid was sent to the Persian Empire with an army consisting of 18,000 volunteers to conquer the richest province of the Persian empire, Euphrates region of lower Mesopotamia, (present day Iraq). Khalid entered lower Mesopotamia with this force.

He won quick victories in four consecutive battles: the Battle of Chains, fought in April 633; the Battle of River, fought in the third week of April 633; the Battle of Walaja, fought in May 633 (where he successfully used a double envelopment manoeuvre), and Battle of Ullais, fought in the mid-May 633.In the last week of May 633, al-Hira, the regional capital city of lower Mesopotamia, fell to Khalid. The inhabitants were given peace on the terms of annual payment of jizya (tribute) and agreed to provide intelligence for Muslims.After resting his armies, in June 633, Khalid laid siege to Anbar which despite fierce resistance fell in July 633 as a result of the siege imposed on the town. Khalid then moved towards the south, and captured Ein ul Tamr in the last week of July, 633.

By then, nearly all of lower Mesopotamia, (the northern Euphrates region), was under Khalid's control. Meanwhile, Khalid received a call for relief from northern Arabia at Daumat-ul-Jandal, where another Muslim Arab general, Ayaz bin Ghanam, was being surrounded by rebel tribes. August 633, Khalid went to Daumat-ul-jandal and defeated the rebels in the Battle of Daumat-ul-jandal, capturing the city fortress. On his journey back to Mesopotamia, Khalid is said to have made a secret trip to Mecca to participate in Hajj.

On his return from Arabia, Khalid received intelligence entailing a concentration of a large Persian army and Christian Arab auxiliaries. These forces were based in four different camps in the Euphrates region at Hanafiz, Zumail, Saniyy and the largest being at Muzayyah. Khalid avoided a pitch battle with a large united Persian force and decided to attack and destroy each of the camps in a separate night attacks from three sides. He divided his army in three units, and attacked the Persian forces in coordinated assaults from three different directions during the night, starting from the Battle of Muzayyah, then the Battle of Saniyy, and finally the Battle of Zumail in November 633 AD.

This string of Muslim victories curtailed Persian efforts to recapture lower Mesopotamia and left the Persian capital Ctesiphon unguarded and vulnerable to Muslim attack. Before assaulting the Persian capital, Khalid decided to eliminate all Persian forces from the south and west, and thus marched against the border city of Firaz, where he defeated a combined force of Sassanid Persians, Byzantine Romans and Christian Arabs and captured the city's fortress during the Battle of Firaz in December 633.This was the last battle in his conquest of lower Mesopotamia. While Khalid was on his way to attack Qadissiyah, a key fort on the way to Ctesiphon, he received a letter from Abu Bakr and was sent to the Byzantine front in Syria to assume the command of Muslim armies with the intent of conquering Roman Syria. During his stay in Iraq, Khalid was also installed as military governor of the conquered territory.

Invasion of the Byzantine Empire
Further information: Byzantine–Arab Wars

Map detailing Rashidun Caliphates invasion of the Levant.
After the successful invasion of the Sassanid Persian province of Iraq, Caliph Abu Bakr’s sent an expedition to invade the Levant (Roman Syria). The invasion was to be carried out by four corps, each with its own assigned targets. The Byzantines responded to this threat by concentrating their units at Ajnadyn (a place in Palestine, probably al-Lajjun) from different garrisons. This move tied down the Muslim troops at border regions, as with this large force at their rear, Muslim armies were no longer free to march to central or northern Syria.[62] Muslim forces apparently were too small in numbers to counter the Byzantine threat, and Abu Ubaidah ibn al-Jarrah, the chief Muslim commander of the Syrian front, requested reinforcements from Abu Bakr. The latter responded by sending reinforcements led by Khalid.

There were two routes towards Syria from Iraq, one was via Daumat-ul-Jandal (Now known as Skaka) and the other was through Mesopotamia passing though Ar-Raqqah. Since the Muslim forces in Syria were in need of urgent reinforcement, Khalid avoided the conventional route to Syria via Daumat-ul-Jandal because it was a long and would take weeks to reach Syria. He also avoided the Mesopotamian route because of the presence of Roman garrisons in northern Syria and Mesopotamia.Engaging them at the time when Muslim armies were being outflanked in Syria, was also ruled out since it would mean fighting on two fronts. Khalid selected a rather shorter route to Syria which unconventionally passed though the Syrian Desert.He marched his army though the desert, where traditions tells that his soldiers marched for two days without a single drop of water, before reaching a pre-decided water source at an oasis. Khalid is said to have solved the water shortage issue using a Bedouin method. Camels were made to drink water after intentionally denying them water for a lengthy time period, encouraging the camels to drink a lot of water at one time. Camels have the ability to store water in their stomach which in turn could be obtained by slaughtering them when necessary. Muslim troops rode entirely on camels and this method became an effective one for the Muslim army.

Map detailing the route of Khalid ibn Walid's invasion of Syria
Khalid entered Syria in June 634 and quickly captured the border forts of Sawa, Arak, Palmyra, al-Sukhnah, al-Qaryatayn and Hawarin. The latter two were captured after the Battle of Qaryatayn and the Battle of Hawarin. After subduing these desert forts, Khalid's army moved towards Bosra, a town near the Syria-Arabia border and the capital of the Arab Christian Ghassanid kingdom, a vassal of the eastern Byzantine Empire. He bypassed Damascus while crossing a mountain pass which is now known as "Sanita-al-Uqab" ("the Uqab pass") after the name of Khalid's army standard. On his way at Maraj-al-Rahat, Khalid routed a Ghassanid army in the brief Battle of Marj-al-Rahat.

With the news of Khalid's arrival, Abu Ubaidah ordered Shurhabil ibn Hasana, one of the four corps commanders, to attack the city of Bosra. The latter laid siege to Bosra with his army of 4,000 men. The Byzantine and Christian Arab garrison which outnumbered Shurhabil's forces, made a sally and were likely to annihilate them when Khalid's cavalry arrived from the desert and attacked the rear of the Byzantine forces, relieving Shurhabil.The garrison retreated to the city's fortress. Abu Ubaidah joined Khalid at Bosra and Khalid, as per the caliph's instructions, took over the supreme command. The fortress of Bosra surrendered in mid-July 634, effectively ending the Ghassanid dynasty.[66] After capturing Bosra, Khalid instructed all the corps to join him at Ajnadayn where they fought a decisive battle against the Byzantines on 30 July 634. Modern historians consider this battle to have been the key in breaking Byzantine power in Syria.

Defeat at the Battle of Ajnadayn left Syria vulnerable to the Muslim army. Khalid decided to capture Damascus, the Byzantine stronghold. At Damascus, Thomas, son-in-law of Byzantine Emperor Heraclius, was in charge of the city's defense.Receiving intelligence of Khalid’s march towards Damascus he prepared the city's defences. He wrote to Emperor Heraclius, who was at Emesa that time, for reinforcement. Moreover, Thomas, in order to delay or halt Khalid's advance and to attain time to prepare for a siege, sent his armies to move forward. Two of his armies were routed first at Yaqusa in mid-August and the other at Maraj as-Saffar on 19 August. Meanwhile, Heraclius' reinforcements reached Damascus before the other column of Heraclius reached the city which Khalid laid siege to on 20 August. To isolate Damascus from the rest of the region, Khalid placed the detachments south on the road to Palestine and in north at the Damascus-Emesa route, and several other smaller detachments on routes towards Damascus. Heraclius' reinforcements were intercepted and routed by Khalid at the Battle of Sanita-al-Uqab 30 km from Damascus.

The route of Khalid ibn Walid's invasion of Syria.
Khalid led an assault and conquered Damascus on 18 September 634 after a 30-day siege. According to some sources, the siege is purported to have lasted some four or six months.Emperor Heraclius having received the news of the fall of Damascus, left for Antioch from Emesa. Khalid's cavalry attacked the Byzantine garrison of Damascus, which was also heading towards Antioch, catching up to them using an unknown shortcut, at the Battle of Maraj-al-Debaj,150 km north of Damascus. Abu Bakr died during the siege of Damascus and Umar became the new Caliph. He dismissed his cousin Khalid from his command and appointed Abu Ubaidah ibn al-Jarrah the new commander in chief of Islamic forces in Syria. Abu Ubaidah got the letter of his appointment and Khalid's dismissal during the siege, but he delayed the announcement until the city was conquered.