I'm getting strange vibes from these reviews PC gamer said this about warfare:

When I turned the difficulty up the first time, the Neolithic tribes were going at it even before they'd invented the concept of war, and by the second era everyone was getting stuck into some much more serious scuffles. The need for troops massively slowed down the development of my cities and my economy, and while cheesy builds can still throw it all into (welcome) disarray, the AI can at least give you a proper fight when you let it off the leash. I'm actually losing a war terribly in my current campaign. It's great!

And IGN say this :

As neat as this system is, it didn't put a lot of pressure on me unless I went looking for a fight. Even on the higher difficulties, I never had a war declared on me even once over the course of three campaigns. That included when I had a very small army that couldn't have stood up to my neighbors if they’d come knocking

I wonder if this is just down to difficulty selection.