Firstly, early medieval Arabs should not be considered so backward in comparison with their neighbors. The first states of the Arabian Peninsula were contemporaries of Ancient Egypt and Assur and were far ahead of the creators of future world empires – Persians and Romans.
Secondly, they were always quite good fighters and one on one pulled against the phalanx, and against the legion, and against cataphracts. At least, they could defeat them on their territory or on a foreign land, if it turned out to take advantage of their advantage in mobility.
In addition, by the time the Prophet Muhammad arrived, they had a very serious economy, as much as possible in the harsh conditions of the Arabian deserts. The endless wars of Rome and Persia caused the decline of both empires, and the trade routes that used to stretch across their territories were now bypassed and passed right through the Arab lands.
The domestication of camels and the development of shipping in the Red Sea made it possible to ensure the transportation of goods to the west from Africa and India, which brought enormous profits to reasonable Arab leaders. And the inability of the leading powers of that era to conquer their distant and desert-protected possessions made it possible not to fear beyond their own borders.
And although at the beginning of the 7th century, due to the war with the Ethiopians, the south of Arabia fell into many small estates, the peninsula was literally strewn with rich city-states, each of which had an abundance of camel caravans and merchant fleets. And in the north and north-west there were militant centralized Arab states of the Lahmids and Gassanids, who were clients of the Persians and Romans.
Muhammad, who relied on a new ideology, managed to quickly unite the rich southwest with the help of war and diplomacy, as a result of which he received at his disposal an economic basis for further expansion. He did not have time to take advantage of his opportunities, but it turned out to be in full with his successors.
The first to fall were Aramaic-speaking Byzantine provinces of the Levant and Syria. They almost did not feel a kinship with Greek civilization and, in addition, adhered to a different form of Christianity than the one preached by the state Orthodox Church. Moreover, long wars have created poverty and devastation here, and Islam, which promised universal equality, very attracted the practically powerless lower classes. Syrian Christians not only did not show much resistance to the Arabs, but also gave them their fleet to conquer Egypt.
The situation in Persia was somewhat different. Persian ideals demanded that the ruler personally lead his troops into battle. This meant that if this single battle was lost, then the fate of the entire war was decided. And the Arabs won this only battle, starting the conquest of Mesopotamia.
The first caliphs were extremely ambitious and successfully combined the cultural and scientific achievements of the Gentiles, Zoroastrians, Jews and Christians. They pursued a policy of constant territorial expansion, and after the death of Muhammad they broke out of the Arabian Peninsula and began to conquer neighboring lands. Each successful victory brought more wealth and power, and also allowed the Arabs to use the civilizational achievements of other nations. Arab civilization has transformed and become much more complex than it was originally.
And the new mentality that arose because of the new faith, allowed to maintain the onslaught of east and west for several centuries. The result was a synthetic empire that the world had not seen before. She took the best from the culture of west and east. In the 8-9th centuries, Arabs were no longer those Arabian nomads who were content with a handful of dates and spent the night right in the desert next to their horses. They became the creators of a great civilization, which greatly advanced all of humanity.