The power that topples the Sassanids comes from an unexpected source.

The Iranians know that the Arabs, a tribally oriented people, have never been organized under the rule of a single power and are at a primitive level of military development.

The Iranians also know of the Arabs through their mutual trading activities and because, for a brief period, Yemen, in southern Arabia, was an Iranian satrapy.

Events in Arabia change rapidly and dramatically in the seventh century CE when Muhammad, a member of the Hashimite clan of the powerful Quraysh tribe of Mecca, claims prophethood in 612 and begins gathering adherents for the monotheistic faith of Islam that had been revealed to him.

Within one year of Muhammad's death in 632, Arabia itself is secure enough to allow his secular successor, Abu Bakr, the first caliph, to begin a campaign against the Roman (Byzantine) and Sassanian empires.

The conversion of Arabia proves to be the most difficult of the Islamic conquests because of entrenched tribalism.

Islamic forays into Iraq begin during the reign of Abu Bakr.

An army of eighteen thousand Arab tribesmen, under the leadership of the brilliant general Khalid ibn al Walid (aptly nicknamed "The Sword of Islam"), reaches the perimeter of the Euphrates delta in 634.

The occupying Iranian force is vastly superior in techniques and numbers, but its soldiers are exhausted from their unremitting campaigns against the Romans.

The Sassanid troops fight ineffectually, lacking sufficient reinforcement to do more.

The first battle of the Arab campaign becomes known as the Battle of the Chains because Iranian soldiers are reputedly chained together so that they cannot flee.

Khalid offers the inhabitants of Iraq an ultimatum: "Accept the faith and you are safe; otherwise pay tribute. If you refuse to do either, you have only yourself to blame. A people is already upon you, loving death as you love life."