Quite clearly, in a novel that is all about war and one man's response to it in a number of different situations, you can expect to find a number of different quotes concerning war that you could comment upon. I will offer the following two quotes, and hopefully this will give you an example of what you are after so that you can return to the novel and find more. Consider the following quote from Chapter Five:
The men dropped here and there like bundles. The captain of the youth's company had been killed in an early part of the action. His body lay stretched out in the position of a tired man resting, but upon his face there was an astonished and sorrowful look, as if he thought some friend had done him an ill turn. The babbling man was grazed by a shot that made the blood stream widely down his face. He clapped both hand to his head. "Oh!" he said, and ran. Another grunted suddenly as if he had been struck by a club in the stomach. He sat down and gazed ruefully. In his eyes there was mute, indefinite reproach. Farther up the line a man, standing behind a tree, had had his knee joint splintered by a ball. Immediately he had dropped his rifle and gripped the tree with both arms. And there he remained, clinging desperately and crying for assistance that he might withdraw his hold upon the tree.
Note the response of the soldiers to being in war and to the death and violence that accompany it. This quote makes it incredibly clear that there is nothing brave and exciting about being involved in a war. Death, whenever and wherever we meet it, is depicted as a force that returns us to the "bundles" or bags of bones that we essentially are. There is no glory in war.
Consider this quote from Chapter 20, which again supports the idea of war being not a glorious event like Henry thinks it is:
Once the men who headed the wild procession turned and came pushing back against their comrades, screaming that they were being fired upon from points which they had considered to be toward their own lines. At this cry a hysterical fear and dismay beset the troops. A soldier, who heretofore had been ambitious to make the regiment into a wise little band that would proceed calmly amid the huge-appearing difficulties, suddenly sank down and buried his face in his arms with an air of bowing to a doom.
Note what characterises the soldiers. As they are attacked, chaos breaks out. There is no order or control. Soldiers are shown to be "screaming" and overwhelmed by a "hysterical fear and dismay." Ironicaly, it is the soldier who had high hopes of showing the order and control of his regiment who is one of the first to die, as if he were "bowing to a doom." The chaos that overtakes the soldiers appears to be inevitable and unavoidable. War is essentially depicted as an event where anarchy rules and where there is no order.