So the countries that carved up Japan are seen as R, C, F, A, and E.
There's a good chance that R is Russia as it looks like they only took the northern tip of Honshu and possibly took Hokkaido too.
C could be China as they took Kyushu and some other portions of Honshu.
Just not sure what F, A, and E are. F could stand for France since they are one of the most powerful military in the world? Not sure what A is, could be just America, kinda like a nickname for the US or could be Australia for various reasons. As for E, I've got no idea, but could be the name for an International agency since that was mentioned.
Ah, whatever, I'm probably thinking too much. It could be that all these letters are for made up and they'll be introduced later for this manga's worldbuilding. Fun thought exercise though.