I tend to play quite peacefully and not focus on war or military equipement. 

City states seem to go the same road to an extent that is frightening.

Context:

I had a CS as Client State for a long time.
They started to ransack one of my outpost.

I came to put an end to the unexpected behaviour (but I think enough people have complained about it, so I'd let it to that).

They backed and then attacked me, showing me that:

Archers, chariots, spearmen...

They gave me enough science and money to upgrade their troops alone.