The Rashidun Caliphate was the first of the Islamic Caliphates to rise to power in Arabia, and after the battles of Qadisiya and Nehavend, Persia too. A success in its time, the opportunity left at the battle of Yarmuk was one that they failed to pick up on, resulting in the Byzantines being able to restore their position in the Levant and establish an unassailable front.
When the prophet Mohammed led the forces of Medina against the forces of Mecca, it was already very clear that the power he and his new religion, Islam, had established in the area. After Mecca had been taken in 630, Mohammed made it his mission to convert all of the tribes of Arabia. Most proved susceptible, and those that didn't were quickly brought to the heel. By 634, Mohammed's successors had established a strong Caliphate, presiding over all of Muslim Arabia.
The Byzantine and Persian campaigns
In 635, General Khalid ibn al Walid, mustered his forces - and, in the name of Caliph Abu Bakr, led them into the territory of their unsuspecting adversaries. Khalid's attempts came to grief at Yarmuk, giving Emperor Heraclius time to get the required number of reinforcements to his Levantine provinces. However, Khalid learned from his mistakes and in 637 he trounced the Persians at Qadasiya in Mesopotamia and again at Nehavend on the Iranian Plateau in 642. The reasons for his delay were mainly to do with troubles with the Byzantines, but in 640 a shaky peace was signed, promising the Arabian coast up to the port of Jedda to the Byzantines. This pleased Heraclius, who hadn't been expecting anything at all, so the peace was signed on this basis. The Persians in Merv, their sole remaining province, tried to make peace on this basis too. In the end it was only a Kushan uprising that diverted Arab attentions and allowed the Persian principality of Merv its precarious existence.
Troubles in Armenia
The Arabs, pleased with how well they had dealt with the Kushans, left their eastern borders at Afghanistan, establishing a regional capital at Kabul to forestall any pinpricking rebellions. They arrived presently at their Iranian heartland, only to find that the Byzantines had cast a protectorate over Armenia. To the Arabs, this would not do, and they immediately presented their case to the Byzantines. Failing this, they called on the support of the Khazars, who simply weren't interested, then on the Persians of Merv, promising to return the Principality of Tabaristan to them. The Persians immediately accepted, followed by their allies, the remnant Kushans. This all looked very impressive on paper, but, the Byzantines had formed a coalition of their own. The Khazars interest had been revived by the Byzantines, and they joined the Byzantine-Armenian defense against Islam, riding out the Arabs attack. The Byzantines then carried their invasion to the Black Sea's western shore, the Khazars swept round the Northern and Eastern shores, easily subjugating Merv, and making what remained a protectorate. The Arabs were forced to give Tabaristan instead to the Byzantines, leaving defenceless Afghanistan to be overrun by the turncoat Kushans, who also brought neighbouring Pakistan under their suzerainty.
Fall of the Caliphate
After the failure of the campaign against the Byzantine-Khazar alliance, the Arab tribes no longer felt so drawn by the Caliphs temporal power. In 650, Abu Bakr successor was assassinated, ending the the shortlived lived Rashidun Caliphate and replacing it with the Ummayad Dynasty under the rule of Uthman.