Depends, if it's for example land, a war will ensue and in most good NRPs (nation roleplays) there is a stat sheet for cheat nation that includes population, economy size, army active/reserve, mobilization potential etc. at least the ones I played had stuff like this. Otherwise , if two nations try getting indirect control of a nation, for example France and Germany trying to scheme for Belgium, they can try fighting each other and the host (GM), which should be pragmatic, lay out how it happens.

An example would be let's say Russia and Germany , in this scenario, are into a war and Germany launches an offensive against Warsaw. Germany can choose to shore up itd offensive capabilities and Russia can try making defenses and see how effective it is. Some people choose to use dice rolls for this but I'm personally against it. You have a rough estimate of the troops around there and the terrain so you can determine what happens. It won't be 100% realistic ofc.