New suggestion from me.
Power Double Standards (I couldn't find a better word)
I've been reading and thinking some more about the Russo-Japanese War and its surrounding events.
So I suggest that Great Powers and Secondary Powers could, when warring with an equal or higher ranked Power, walk into the territory of states that are not Great Powers or Secondary Powers. The idea is that when this happens the state with ignored borders get a choice to either resist with force or stand down. This could also come for an Infamy cost that represents public and diplomatic blowback against such a violation of borders.
Resist with force creates a war with the Power that walked across the border and also offer a chance to seek an alliance with the states at war with this Power.
If you stand down you lose alot of prestige and other stuff like increased militancy, loss of legitimacy, support for the parties in the government and so on. And the Power that you stand down towards drastically increase their influence over you, as well as can use you ports and territory for supply lines and you can be blockaded and such by the other side, even if not at war.
The basic idea of this would be to make wars more unstable, as new combatants can easily be drawn in, while also making it desirable for smaller nations to play the diplomatic game more as to shield them from expedient occupation by a nearby Power engaged in a major war.
Historical Examples: My main examples are the Japanese invading Korea in 1904 to secure bases for a war with Russia fought in, unless I'm wrong, formally Chinese territory occupied by Russia, with civilian population being treated pretty horribly in the war between Russians and Japanese.
And of course Germany waltzing into Luxemburg and Belgium in 1914.