Also, here is a sketch of what I would envision a simple set of hostilities / alliances would look like. I think this is deep enough to still allow for some interesting new strategy, while still being simple / hard to accidentally misinterpret.
You relationship with each country can be in one of 6 states:
1) War: All is fair. Do whatever you want to their territories. Once you enter war with another country any ally (State 5) you have, will automatically be Hostile (State 2) with the country you are entering war with until either they also declare war, or you cease war (either through diplomacy or are defeated).
2) Hostile: You may not directly attack said country, however you can support moves into their territories or convoy allied (State 5) units into their territories.
3) Neutral: This is you relationship with all countries at the start of the game. Essentially you two can't interact other than bouncing in neutral (uncolored) territories.
4) Friendly: You may support hold units of the other country. You may also convoy their units to a neutral zone, and support their units to a territory they own, or is neutral.
5) Allied: Same as friendly except you can also help (support move / convoy) into hostile or warring countries.
?) Allied + Hostile: When two of your allies declare war on each other you become both allied and hostile with both countries. While in this state, you can only act as friendly to either one unless you are also warring/hostile with a fourth party, in which case they both are treated as allies concerning the fourth party. (Until you decide to pick a side and drop your relationship of allied down)
* A country's territories are defined as anything they have a unit in, or is colored their color.
* To change status towards a higher number both parties must agree. To change it to a lower status only one party need make it known.
* Changes in status must happen X hours before the turn moves
* Spring 1901 is extended an extra X hours to allow spring attacks to be feasible.
Personally I think it's elegant if X is just the phase length, but I can see why you may want it to be shorter (or for very politically slow games, an in-game year).
Not saying we should go with this, but offering it up as one potential starting point.