And should they? Did they have a lot of room to expand still? Did they have enough cities already? Did they have unhappiness problems, or close to it? Did they consider you as a friend, or have no reason to attack, other than "just" for the sake of attacking (expanding)? Were they civs that have generally more friendly flavours, or those that like to go and eat a civ for every major meal? Are you on higher difficulties? Did you ignore their request? Are you competing for City-States / Wonders? Were there others that they had planned attacking, or maybe considered you to be a help against someone else? It's an equation with a lot of variables
I'm not against an aggressive AI, but it should be reasonable. I had my southern part whopped twice now by the AI, one joint war when after they were done finishing my poor neighbour they turned to me, other one Rome being Rome and expanding in the early game (and me being comfy and ignoring my army)
All fair and square. (but at least I managed to stop two Mongol invasions in my current game yay!) And all of that on Emperor / Huge map, what's unless I make some grave mistakes is usually pretty comfortable for me.
Not to mention that a hyper-aggressive AI can put itself into a worse position: if they have DPs, the human has just bypassed it. A warring civ is also a more potential attack for others to attack (unless I'm mistaken, Recursive can correct me), and there is also war weariness, diplomatic penalties, etc. A slight increase probably wouldn't hurt if they have clear objectives against you, but these also have to be taken into consideration.
The point I'm trying to make is that just because you (or the AI) can do something, doesn't necessarily mean you (or the AI) should do it. At some degree, I think you should be able to overcome your military weakness by diplomatic means (however, if they hate you or have a clear reason to invade you, could do it easily and they don't do it, now then that's definitely something to adjust). And let us not forget to draw conclusions only after a fair number of games (I'm not saying you didn't, just an advice that I also have to tell myself
).
Poor Recursive is adjusting aggressivity back and forth between each patch
(but at least hopefully we're getting closer to the optimal one by each step
)
I feel the AI to be quite alive and dynamic (just an example from the same game: Ethiopa and I have some beefs regarding religion and CSs, but against the Mongol threat we made DoF, DP, and so on. That's exactly what I call as reasonable diplomacy!) It was really good before, and now I feel like some really good steps have been taken to make it even better!