This is something that happens to me sometimes in the early game. I am playing a game where my neighbor dow'd me because relations are utterly terrible, but they don't seem to be sending any troops or fleets so far. In fact, in the diplomatic screen I get a ++ to relations because they I am outside of their range or something. If that is the case then why go to war? In fact I am not even sure if they are able to even build transports though I can't confirm them since the graphs show they have more advanced tech than me.
Bonus question: Does the AI in general now assemble powerful fleets and armies and actually go on the offensive? I almost never saw that in the past. This is my first new game in a considerable while.