During WWII(?) a group of Japanese and US military are hiding out in a temple, and are transported to another world. In this other world they are slowly able to fly and shoot fire from their hands. They learn that in this world the gods and demigods exist among the people and they are at war. They enlist in this war on the same side, and fight for the "good" gods. At some point the gems that they and other sorcerers use to focus their magical energies are used as bombs, and they fly in bomber formation to drop them on the enemy.
The Crystal Warriors by William R. Forstchen and Greg Morrison
From review on goodreads.com:
A group of American soldiers are being chased by some Japanese soldiers somewhere in China during World War Two. They stumble across an ancient temple, and both the Americans and the Japanese are pulled through to another world.