Divide the map into regions that cost movement points.
in order to go to war, you have to be able to reach a player within a certain number of moves. Or in order to trade, you must be within trading distance.
Then allow players to own these regions.
ownership of a region gives one bonus. The ability to deny other players access in or out of the region.
While the owner of a region, anyone can attack you, but you can only attack people in your war range.
to become the owner of a region just go to war. The war type would be "Regional Hegemony."
If a small nation wishes to simply get out to trade with another nation but is blocked, have a war type called, "Resist the Blockade." one successful attack allows the player to perform their desired action.
Below is a simple map of the world (I am by no means an artist). most of the borders follow mountain ranges or rivers. It also includes the ocean in the territories in case people are trying to be Atlantis. 4 military moves should be enough to get anywhere on the map without blocks. 5 would allow for some wiggles room. I think between 6-8 moves should be a good number for trading.
Or have a grid with squares as regions. That might be simpler but still is the same idea.