Each dynasty has a certain amount of provinces and lands, and no more
This is because the group to which a given dynasty belongs and the people who support and establish it, must of necessity be distributed over the provinces and border regions which they reach and take into possession.
Only thus is it possible to protect them against enemies andto enforce the laws of the dynasty relative to the collection of taxes, restrictions, 41 and other things. When the (various) groups have spread over the border regions and provinces, their numbers are necessarily exhausted.
This, then, is the time when the territory (of the dynasty) has reached its farthest extension, where the border regions form a belt around the center of the realm. If the dynasty then undertakes to expand beyond its holdings, it(s widening territory) remains without military protection and is laid open to any chance attack by enemy or neighbor.
This has the detrimental result for the dynasty of the creation of boldness toward it and of diminished respect for it.
On the other hand,) if the group is a very large one and its numbers are not exhausted when distributed over border regions and territories, the dynasty retains the strength to go beyond the limit (so far reached), until its expansion has gone as far as possible.
The natural reason for this (situation) lies in the fact that the power of group feeling is one of the natural powers. Any power resulting in any kind of action must proceed in its action in such manner.
A dynasty is stronger at its center than it is at its border regions. When it has reached its farthest expansion, it becomes too weak and incapable to go any farther. This may be compared to light rays that spread from their centers, or to circles that widen over the surface of the water when something strikes it.
When the dynasty becomes senile and weak, it begins to crumble at its extremities. The center remains intact until God permits the destruction of the whole (dynasty).
Then, the center is destroyed. But when a dynasty is overrun from the center, it is of no avail to it that the outlying areas remain intact. It dissolves all at once.
The center is like the heart from which the (vital) spirit 43 spreads. Were the heart to be overrun and captured, all the extremities would be routed. This may be observed in the Persian dynasty. Its center was al-Mada’in (Ctesiphon).
When the Muslims took over al-Mada’in, the whole Persian empire dissolved. Possession of the outlying provinces of the realm was of no avail to Yazdjard.
Conversely, the center of the Byzantine dynasty in Syria was in Constantinople. When the Muslims took Syria away from the Byzantines, the latter repaired to their center in Constantinople. The loss of Syria did not harm them. Their rule continued there without interruption until God permitted it to be ended 4 4
Another example is the situation of the Arabs at the beginning of Islam.
Since they were a very large group, they very quickly overran neighboring Syria, ‘Iraq, and Egypt. Then, they kept on going, into Western India (as-Sind), Abyssinia, Ifrigiyah, and the Maghrib, and later into Spain. They spread over many provinces and border regions, and settled in them as militiamen.
Their numbers were exhausted by that expansion. No further conquests could be made by them, and the Muslim empire reached its farthest extension. Those borders were not passed, but the dynasty receded from them, until God permitted it to be destroyed. The situation of later dynasties was the same. Each dynasty depended on the numerical strength of its supporters. When its numbers were exhausted through expansion, no further conquest or extension of power was possible.