This is a tricky question - how can we create a system in which we at one hand can start large multi-national operations like overlord without taking over all units of our allies.
Possible solutions:
- You can request expeditionary forces from allies - but not like in HOI III where you would get random amount dropping in, but with a request for a certain number of divisions, aircraft and ships. The allies can either agree, outright decline the request or make a counter-proposal with fewer units. The number of troops allies will be willing to give could depend on factors like relations and their own needs of troops. Also allies can always call back their troops (perhaps with a 1-month waiting time that your front doesn't collapse immediately) if they see the need to do so. If possible expeditionary forces could also be limited to a theater. The USA are maybe willing to lend Britain troops for an invasion of Italy, but they won't like it if Britain sends those troops to India instead.
-Expeditionary forces could be tied to battle plans. A coordinated battle plan can include troops from other nations, and if these nations agree on the plan, they hand over the involved troops as expeditionary forces. Of course there are still some open points: For example, is it possible to add units which are not yet available at the front to a battle plan? (This could be an issue for planning something like overlord, where you need to get the allied forces to Britain first before drawing the exact plan.) And what will happen if the plan was executed successfully, will the forces remain under the players control?
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