What pain did the Giver ask Jonas to take from him in chapter 15?

One day when The Giver is obviously suffering, Jonas offers to help him, and The Giver asks him to take some of the pain. The memory is of war, and Jonas finds himself on a battlefield next to a dying boy who asks him for water. Jonas's own arm is shattered, but he uses his other arm to give water to the boy's lips. However, the boy dies, and the cries of injured men and horses remain, while he hears cannons in the distance and waits in pain listening to the sounds of war. When Jonas can no longer bear it, he opens his real eyes and mentally returns to the bed. The Giver asks him for forgiveness.