The Military Horizon is a term John Keegan wrote about in his books The Face of Battle and A History of Warfare. Its an anthropological term, describing the point where tribal warfare gives way to sustained campaigns of military force. That is, nearly any culture can wage war, but does it have the logistics, skill sets, and technologies to form a military.
Or simply: does the culture send a crowd (as Keegan put it) to fight a war or send a military?
Tribal warfare involves raids. The warriors leave the tribe for a few days at most and come back when they’ve run out of food or brought back plunder. Perhaps the might be to force an enemy tribe out of a territory from time-to-time, but tribes lack the sustained logistics to do this.
A culture rises above the military horizon when it can form a military. Usually this concides with the same conditions needed to form a city-state. That is: social hierarchies, divisions of labor, agriculture, and so on. While the soldiers are off fighting the enemy, the farmers at home raise food, while some form of supply chain sends the food (and perhaps additional reinforcements) to the soldiers in the field.
Within a generation Philip II of Macedon, for example, took crowds of mountain herders and trained them to form phalanxes and use the sarissa. He brought his people above the military horizon. Then his son, Alexander the Great, used this new military force to conquer most of the known world.
To put it another way, nearly any culture can produce warriors, but it takes certain conditions to produce soldiers–that is, professional warriors who fight for pay, whether its money or land.
I’m being overly simplistic here. Any military historian with enough knowledge and research will know Keegan proposed two military horizons. And not all cultures conform to the military horizon theory and yet build mighty empires without cohesive militaries–The Mongols, especially in their early conquests, being a notable example.
When it comes to the military history of religion, it should be obvious that religious military violence stems from the union of church and state, above the military horizon. Yet even the military horizon isn’t always a clear demarcation?
Is it possible for a zealot to believe they are part of God’s army? And thus the concept of the military horizon becomes distorted or inverted?
Certainly. See The Book of Joshua in the Old Testament or the accounts from the First Crusade, where the warriors believed God would sustain them and provide the logistics to conquer their enemies.