How did the military profession come into being?

This question is easy to answer. In primitive society, men had to take part in the usual hunting activities, which greatly exercised the unity and cooperation of tribal members, because large fierce animals depended on everyone. With the increase of tribal population, the friction between neighboring tribes is inevitable, so the earliest soldiers appeared. Of course, soldiers were not professional at that time. They usually take part in daily work and organize themselves when fighting the enemy. With the development of tribes into tribal alliances, there are more and more wars between different alliances, so someone has to be specially trained in military struggle. Organizations and other activities, these people are the earliest professional officers, so the earliest professional soldiers were born. Except for a few officers, most soldiers are organized temporarily, and they usually take time to participate in training, which is unprofessional, while the guards of kings and nobles are obviously professional.

With the development of society and the expansion of war scale, the rulers felt the importance of elite troops, so they set up large-scale professional elite troops one after another, like the "soldiers" set up by Wei in the Warring States Period.

The "undead armies" of the Persian Empire are all elite professional troops with a large number.

This is how the military profession developed.