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Inspired by this thread:
https://www.alternatehistory.com/discussion/showthread.php?t=161167
What alliance is truly unbeatable?
Though this is in After 1900, I don't mind alliances prior to it.
The alliances can be ASB for example:
1979 - USSR + USA
Unbeatable?
Unfortunately nukes kind of spoil it.
Unbeatable would be that they can be at war with the entire world and not lose. (We can perhaps change that definition if it's too ASB)