The Rise of Islam by Georgio Antoniou, Library of Rhomania, Rhomania
The Battle of Yarmouk
Prelude
Prior to the rise of Islam, the Rhomanians and Sassanids were involved in vicious wars. The Sassanid attack successfully was able to conquer land as far as Egypt and get deep into Anatolia. This attack, despite being an astounding success, weakened the empire. This opened a passage for a Rhomanian counterattack. Under Heraclius, all lost territory was reconquered. This was wildly successful. The Rhomanians then conquered Mesopotamia. This brought them in close proximity to Ctesiphon, the capital of the Sassanid Empire. King Khosrau II was now subject to a coup. This brought the beginning of the Sassanid Civil War.
The land of Arabia was ignored. They were though of as nothing more as desert. In the trade city of Makkah, a merchant named Muhammad founded a religion. This religion was Islam. Unifying the peninsula under his control, the Arabs were becoming a powerful people. However, soon after this, Muhammad resigned. His friend Abu Bakr became the caliph of Islam. However, there were rebels who did not want him to become caliph. These tribes rebelled in a series of wars known as the Ridda Wars. However, Abu Bakr was able to bring these rebels under his yoke.
Following these wars, the Arabs began a series of campaigns to conquer Mesopotamia. These successful campaigns ruined Persian morale, being beaten by silly "barbarian Saracens." Following this, the Arabs began to look to Arab territory to conquer. However, now they looked to Rhomania and the Levant to conquer. However, despite good tactics, they sent too few soldiers. As a result, more soldiers were sent in. In July 634, the Rhomanians were defeated and the Levant fell to the Arabs.
The Counterattack
Rhomania under Heraclius sent troops into Syria. The Persians sought an alliance with Rhomania and Heraclius married Yazdegerd's daughter. The plan was, when Rhomania attacked Syria, Yazdegerd III, emperor of the Sassanids, was to attack the Arabs simultaneously. This was a success. The Arabs were stretched by this attack. Heraclius won at Yarmouk in an amazing victory, while the Persians failed in Mesopotamia. The Arabs were forced out of the Levant for now. They looked elsewhere for conquests for the time being. However, the Rashidun attack against the Persians had begun. Within a month, they were annexed and defeated by the Arabs. To the Arabs, it seemed that conquering the Levant was against Allah's will. From that point on, they expanded to the east.