Quote Originally Posted by Razor1952
So gaol #2 is not training too many troops.
No. The AI makes invasion decisions based on:

1. Strength of your 'frontline' troops (those manning the border).
2. Strength of your 'reserve' troops (those sitting further back ready to respond to any incursion.
3. Production capacity of your empire compared to theirs (how many units you crank out in a turn).

Having a weak frontline invites those 1-2 unit stack sieges on your cities. Weak reserves invites an invasion. Having a 'weak' production rate invites a massive buildup of AI troops, followed by an invasion. This last factor is tricky because apparently your production rating is based on how many units you produce a turn. I hardly produce anything once my garrisons and field armies are at full strength, so the AI usually thinks I'm weak.

So if you only have minimal garrisons and skimp on the troops, expect more raids and outright invasions by the AI. Being peaceable doesn't pay. Parking a full stack or two of elite troops right on each border with another stack in reserve can sometimes deter the AI from rash decisions. It also allows YOU to grab some new real estate across the border should the opportunity present itself.