(1) The evidence for Jesus of Nazareth and the evidence for Muhammad of the Quraysh are two separate discussions. We have four canonical biographies of the former within less than a century of his recorded death. For the latter we have one accepted biography from almost 150 years after his recorded death.
(2) Various movements of conquering peoples have occurred in history without having a distinct religious motive or a clear originator. The Khamag Mongols had no religious agenda and had already begun their enormous empire building project before Genghis Khan was born. Likewise the French conquest of Europe had begun before Napoleon's name was known. The Roman Empire had no clear progenitor or religious basis and the Persian conquest of most of the known world was also spontaneous and non-ideological.
History shows that peoples conquer first because they can, and invent supporting ideologies and align behind leaders later on in order to consolidate and legitimize their gains.
I am not saying that Spencer's case is unassailable, only that your objections are not well-supported.
The Mongols, and all related people of the steppe, showed a continuous history of raids, invasion and conquest for thousands of years.
The Arabs had a history similarly thousands of years old. Lots of raids, but few or no successful conquests. Then a century with one of the greatest conquests in history, then a return to centuries of no conquest. For that matter, a history of being invaded.
My question is simply what can account for the discrepancy.
I think the simplest explanation is the birth of a new religion.
I agree the evidence for Jesus is not identical to that for Mohammed. I merely pointed out that similar objections to whether he really existed have been made. Which they have.
I plan to read the book. However, my initial impression is that of the old science saw, “Absence of evidence is not evidence of absence.”
IOW, the lack of evidence does not prove he didn’t exist, only that the evidence he existed is minimal.
Scientists and historians have for centuries now been claiming that all sorts of mythical characters had no basis in fact. Quite often as additional evidence is found it shows the guy really existed and may have been exaggerated. But very seldom that there was no basis in a real person.
The Xia and Shang dynasties in China, for instance.
Not completely. The Mongols were not united before Genghis Khan. There were various groups: naimans, Kara-khitan etc. Chinghiz Khan did unite them under the idea of the sky-Lord and battle against the decadent city-dwellers.
Likewise the French conquest of Europe had begun before Napoleon's name was known. -- the French conquest of Europe was quasi-religious in the Republican sense, they had a mission. Also, France at that time was 25% of the population of Europe, and heavily militarized due to the revolution. Even a highly militarized, but small state like Prussia could not stand up to this rag-tag army that used unconventional tactics.
incidently from 1800 to 1900 the population of France remained nearly stagnant while England's population nearly quadrupled and Germany's quadrupled. Some suggest this was due to the French learning different methods of birth-control in their travels east.
The peoples of the steppe had a several thousand year history of conquest down into the settled areas of China, India, Persia and the Middle East by the time Genghis came along. Intermittently a great leader would arise on the steppe, unite the tribes and conquer down into one or more of the areas. Genghis was not an innovator, only the most successful of all the leaders to arise from the steppe.
The steppe peoples lived a similar lifestyle from Hungary to Korea. Tribes coalesced and splintered easily. There was nothing resembling "nations" in the sense we use the term. A long period of good rains would result in significant growth in the herds and population of the nomads. When drought inevitably returned, they fought each other for pasture or turned outward for conquest of the farmers.
The settled nations on the perimeter of the steppe defended themselves primarily by divide and conquer, bribing the tribes to fight each other rather than unite and attack the civilized areas.
The basic military fact was that before the introduction of effective firearms mounted archers properly used were invincible, at least on reasonably suitable terrain. Any force powerful enough to defeat them was too slow to catch them.
My point is that the Arabs had no such history of conquest. This is probably at least partially because the Arabian peninsula is not nearly as large or as suitable for supporting large tribes of nomads as the Eurasian steppe. Much of it is true desert, not steppe.