Interestingly enough, agrarian societies being invaded (and then ruled) by nomadic tribes has been a recurring phenomenon in the past two millennia (by the Arabs, the Turks, the Mongols, etc.)

I think that's something a bit different though - a later era, with some of these nomadic tribes being more inherently inclined to war and violence, and "war technology" such as horses, advanced bows, etc, playing a role.

The suggested universal dynamic of neighbors in conflict is different from that of well equipped, well trained, expansionist cultures preying on everyone.