The Rashidun Caliphate had its inception with the advent of Islam. It was situated amidst two of the most formidable empires of the known world: The Byzantine Empire and The Sassanid Empire.
But within 30 years, the Caliphate overwhelmed both the empires. The Byzantine Empire lost more than 60% of its territory to it. The Sassanid Empire was completely annexed, and the dynasty ended.
Here is a list of battles which the Caliphate armies won during these years Against Persia:
First wave (633) , under Khalid Ibn Walid,(about 18000 troops)
- Battle of Chains
- Battle of River
- Battle of Walaja
- Battle of Ullais
- Siege of Hira
- Battle of Ein ut Tamr
- Battle of Muzieh
- Battle of Sanni
- Battle of Zumail
- Battle of Firaz
- Second Wave
In 635 Yazdgerd III sought alliance with Emperor Heraclius of the Eastern Roman Empire. Heraclius married his daughter (or, according to some traditions, his granddaughter) to Yazdegerd III, an old Roman tradition to show alliance. While Heraclius prepared for a major offence in the Levant, Yazdegerd, meanwhile, ordered the concentration of massive armies to pull back pull back the Muslims from Mesopotamia for good. The goal was well-coordinated attacks by both emperors, Heraclius in the Levant and Yazdegerd in Mesopotamia, to annihilate the power of their common enemy, Caliph Umar. While Heraclius launched his offensive in May 636, Yazdegerd was unable to muster his armies in time to provide the Byzantines with Persian support. Umar, allegedly aware of this alliance, capitalized on this failure: not wanting to risk a battle with two great powers simultaneously, he quickly moved to reinforce the Muslim army at Yarmouk to engage and defeat the Byzantines. Meanwhile, Umar ordered Saad to enter into peace negotiations with Yazdegerd III and invite him to Islam to prevent Persian forces from taking the field. Heraclius instructed his general Vahan not to engage in battle with the Muslims before receiving explicit orders; however, fearing more Arab reinforcements, Vahan attacked the Muslim army in the Battle of Yarmouk in August 636. Heraclius's Imperial army was routed.
Under Sa`d ibn Abi Waqqas
- Battle of al-Qādisiyyah effectively ending Sassanid rule west of Persia proper. This victory is largely regarded as a decisive turning point in Islam's growth: with the bulk of Persian forces defeated, Saad later conquered Babylon, Koosie, Bahrahsher and Madein. Ctesiphon, the Imperial capital of the Sassanid Empire, fell in March 637 after a siege of three months.
Under Abdullah ibn Muta'am
- Battle of Jalula
- Siege of Tikrit
- Battle of Khaniqeen
- Siege of Hulwan
Qa'qa sought permission for operating deeper into Persian land, i.e. mainland Iran, but caliph Umar didn't approve the proposal and wrote a historic letter to Saad saying:
"I wish that between the Suwad and the Persian hills there were walls which would prevent them from getting to us, and prevent us from getting to them. The fertile Suwad is sufficient for us; and I prefer the safety of the Muslims to the spoils of war."
- Siege of Susa
Siege of Junde Sabur
- Entry in Persia
After several years, Caliph Umar adopted a new offensive policy, preparing to launch a full-scale invasion of what remained of the Sassanid Empire. The Battle of Nihawand was one of the most decisive battles in Islamic history. The battle proved to be the key to Persia. After the devastating defeat at Nihawand, the last Sassanid emperor, Yazdgerd III, fled to different parts of Persia to raise a new army, with limited degrees of success.
- Battle of Ishafan
- Siege of Ishafan
- Siege of Rey
- Siege of Sistan
- Siege of Kerman
The battles won against Byzantines are even more numerous.
I have typed these great lengths to exactly illustrate my point, the magnitude. Some victories can be called turn of situations, some talent of generals, some psychological victories. But, the magnitude of the phenomenon so large, it can't all be thrown to individual reasons. One significant reason maybe instability in Persia, but still the question stands.
Such conquests, and attaining such largeness, takes long and long years of civilizational strides by nations, to have the economy to support, and mobilizing great manpower, or advances in weaponry, or innovative battle tactics, or producing brilliant generals, or strong fealties to the nation, or all of them. The Romans attained that in few hundred years, the Persians with centuries of continuous buildup, the Greeks, with years of growth of philosophy, the mongols, with centuries, before weaponizing their riding skills.
So the question is: What disruption, or innovation, or disruptive innovation did the the advent of Islam bring, and render on the Arabs, that a nation of nomads went up and annihilated Great Empires, conquered great lengths of Earths in such short period of time (again, some 40 years), even just after emerging as a nation? And going on to win against deep rooted civilizations?