That could be countered by giving no real reason to invade in the first place. If you look at alot of the territory in the game, there's simply no reason to hold it. an example, the desert in north africa. Yes it's nice if it's my colour, but holding it gains me absolutely nothing. So, in the game(and IRL), countries should avoid taking places with no real value.
To illustrate what occurs now(why's are listed in order of importance to my war effort):
if i'm playing italy, almost immediately i send troops to seize albania: why? coal, iron, oil, IC boost
After consolidation of ethiopia/albania I quickly send troops to greece: why? IC, iron, coal
then, Venezuala: oil, IC, coal, iron
after that, as far as natural resources go, i'm set and i start looking for high IC provinces/countries to supply my future war effort in europe.
there should be 4 reasons to invade/protect a province country:
Victory points, IC, resources, strategic position(supply line, choke hold, port etc). Right now the minors are swimming in IC/resources and so unhistorical events occur because they are to the player's advantage. The island of santo domingo/haiti gives 10 + IC, coal and iron. In comparison, this one caribbean island has the same resource/industrial capabilities as most industrialized provinces. If it wasn't worth VP(which it shouldnt be), had 1-2 IC, little iron/coal...I wouldn't be troubled to take it.
The other side that will hopefully be corrected is to make the majors concerned if a country takes a province that is high in one of those 4 areas. An italian invasion of venezuala, should at the very least provoke severe international concern, if not outright war..
anyways, just my thoughts....