I would suggest there are two related factors here. Before significant agriculture some amount of warfare was simply a part of life as usual. This scenario su

I would suggest there are two related factors here. Before significant agriculture some amount of warfare was simply a part of life as usual. This scenario suggests martial competence is fairly ubiquitous, and thus dominating enemies without paying a high cost in blood is difficult (e.g. you might need great political skills to stitch together a powerful local coalition). However once significant cities were established, being skilled at warfare was potentially very lucrative. This means a soldier specialist may be a very rewarding career. Is it really the plough itself, or that the larger cities the plough helped create were a big incentive for professional soldiers to become the ruling class?