Grenade / Alan Gratz.

Enhanced descriptions from Syndetics:

Here it is! The hugely anticipated follow-up to Gratz's NYT bestselling, critically acclaimed phenomenon REFUGEE. This is another searing and heart-pounding look at kids making their way through war. A New York Times bestseller! It's 1945, and the world is in the grip of war.Hideki lives on the island of Okinawa, near Japan. When WWII crashes onto his shores, Hideki is drafted into the Blood and Iron Student Corps to fight for the Japanese army. He is handed a grenade and a set of instructions: Don't come back until you've killed an American soldier.Ray, a young American Marine, has just landed on Okinawa. He doesn't know what to expect -- or if he'll make it out alive. He just knows that the enemy is everywhere.Hideki and Ray each fight their way across the island, surviving heart-pounding ambushes and dangerous traps. But when the two of them collide in the middle of the battle, the choices they make in that instant will change everything.From the acclaimed author of Refugee comes this high-octane story of how fear can tear us apart, and how hope can tie us back together.