I've recently been trying to learn about the formation or decline of some of the more prominent historical nations (England, the Mongol Empire, Rome, etc.) and it seems as though many of the key leaders would absorb soldiers into their ranks that they conquered. In some instances, much of the army in question consisted of soldiers they once fought against and conquered. It got me wondering, how common was this? Were there many instances of it backfiring? It seemed that oaths and allegiances did not really mean all that much in the past. Soldiers would just as soon fight against for their enemy. These did not seem to be mercenaries either, although I guess they were paid in safety and the loot from victories. It just seems strange to me as I can't imagine something like that happening now but seems relatively common place in the past.
It's important to remember that the concept of loyalty to a country did not really exist among the bulk of soldiers during these times. Europe was for the most part not divided into unified countries as it is today, and in the majority of mainland Asia, even the unified China, commoners were still more loyal to whatever small territory they came from. Most soldiers lived in a very small part of an already divided nation, so getting called into a Lord's service was not exactly a matter of "Patriotism" or "Pride," except in specific cases (I.E. The Crusades and La Reconquista in Europe). Part of the reason for this is that people in general were far less connected with the outside world, and they were frequently not educated about the conflicts in which they were fighting. All of these factors cause a lack of loyalty, especially among the poorest soldiers in an army. Sources:
Reynolds, Susan, Fiefs and Vassals: The Medieval Evidence Reinterpreted. Oxford: Oxford University Press, 1994
David Crouch. 2002. William Marshal. Knighthood, War and Chivalry, 1147–1219. Longman. London