With what fear was Henry largely preoccupied for much of the beginning of the novel?

He was worried about fighting at war

Explanation:

Henry was a soldier who had to go to battle, he had heard from veterans that being at war was very hard and all they saw around them was blood, fire and smoke. All these aspects made him grow anxious and to his mind arrived the idea that he could get scared and run from the battle field, this started to be his deeper fear.