Personally, I think it is contradictory to reroll.
Regardless of the result (I want Japan to win), the purpose of using dice is to remove authorial bias (if present), allow for unique twists, and even provide different ideas from what was expected.
If the end result is just going "Nah, that roll didn't pan out", don't even roll the dice and just decide what happens. If the dice roll ends up still being a fumble, but for the other side, why not just re roll that? Why not reroll until they both get a natural crit?
Imo, rerolling is basically saying "I want to have randomness, but only if I agree". My opinion would stand even if the roll was something crazy like "Drakia Invades America" and America Crit Fails while Drakia gets a nat 20. Clearly thats probably the worst possible outcome (I imagine that'd be a harsh peace for sure), but if thats not an acceptable outcome for the story, it shouldn't be given the chance via dice roll.
In the end, however, the choice is yours. I would like for you to do a sort of "Alternate History Novel" cameo in the story about what the first roll was, or vice versa.
Infact, that might be a nice compromise. Maybe see what you come up with for the current roll, then do a re-roll (if others agree with my idea/re-roll), then pick whichever is a better story in your eyes. Then the alternative (first roll or re-roll) can make a cameo as a alternate history novel within the story. (Or something similar.)
Edit: On further consideration, I do believe that my last idea is most interesting, if only to know what the re-roll would be. (I like seeing what my Mulligan would be when doing the 3 randoms thing for sports games etc) But I would much prefer the original roll to stand. And I agree that doing a "and then one side gets distracted" would be a huge cop-out and very anti-climatic.