Long ago, with the Classic game, they had a rule called “Commander In Chief” where you could make multinational attacks. Basically, two or three countries would group some forces together (for example, US and British forces building up on United Kingdom) and at the end of the non-combat move phase of one of the nations, let’s say US, the players would decide to designate the US player “Commander In Chief” and stack 3 US control markers there. On UK’s next turn, they can still move any units in that grouping, but then those units are no longer part of the combined force. Then, on US’s turn, they can move all of the US pieces plus any British pieces that DID NOT move on UK’s turn in a huge combined force attack.
I think when the 2nd edition of the Classic rules came out, they eliminated the “Commander In Chief” rule and I haven’t seen it in any of Avalon Hill’s versions. I always thought it was kind of a cool rule and don’t understand why they did away with it. Maybe they figured out like chompers said that it would be too easy to exploit and give too much advantage to the Allies. After all, in Classic they didn’t have Italy and it was very rare that Germany and Japan could actually help each other.