[Illuminated One]

Hmm, I play on difficult setting and I have a similar problem. however only with certain nations.
If Niefel is in the game it usually goes like this. I expand until I border Niefel. Than I remember the previos game "OMG it's Niefel I have to destroy them", so I attack them early use lots of spies if I can and even take some of their territories.
But they simply don't care their armies keep growing as does their land and sooner or later they return with a really huge army (100 of the smaller giants + a lot of indies). Even if I can defend against that army I takes only a few turns and they are back with an even larger force and other AI nations invade me, too.
So I start another game (again with niefel because I must find a way to defeat them) and exactly the same thing happens.
The same happens to a lesser extent with some other nations (vanheim for instance but that seems to be more random).