This is WAR!!! What region do you want the help of?

Um… Giovanni and team Rocket aren't actually all that powerful…
But Alola… I understand but also don't. Alola is a very powerful region. But… the Tapu do one thing and ONLY that. Protect Alola. Necrozma is technically in another dimension with the other UBs. And Red and Blue were only in Alola to be Battle Tree captains or whatever, they probably went home after that. So you pretty much get… the Captains and Kahunas, maybe Sogeleo and Lunala if Lillie isn't hiding them from war, and most of the Totems are at least partially under the Captain's command so I guess them too.

Kalos because they have experience with war, first of all!
The Ultimate Weapon was not destroyed! It retreated into the ground at the end of X/Y! IT IS STILL THERE. That ALONE is enough to convince me! It has destroyed BOTH sides of a war before.
Zygarde exists to put the ecosystem in balance, and war isn't good for that… if I were to convince Zygarde that my side would end the war faster and restore everything to peace, that's a very powerful ally.
Not to mention the Legendaries of life and death, which are helpful for obvious reasons! They've literally turned wars around in the lore of Pokémon before!
Then we've got Eternal Flower Floette, Mega Evolution, a pretty big load of Mythical and Legendary Pokémon…
Yeah.

Sinnoh region because it has most of my favorite Pokemon.