This is well written! You have a nice grasp of vocabulary and descriptive language. I am going to make a few little suggestions that I feel will strengthen your paper even more ...
Clarify in your introduction that you are contrasting the American depiction of Germans in their propaganda posters because both sides had posters.
a young group of German soldiers
Minor thing here ... I would put "young" in front of "German" instead. They are a young group, but you want to emphasize the youth of the soldiers.
The soldiers depicted in Remarque's novel are young men
The instructions say that the paper should be exclusively in active voice. Man, that is tough and it doesn't always produce smooth sentences! You could change this to the active voice by saying: Remarque depicts the soldiers ...
Despite their lack of age, however, war has quickly robbed them of their mental youth and innocence.
This sentence is a little awkward. Age isn't something I think of people as lacking. They may lack maturity. They may be young. But they don't really lack age. You are also skipping around a bit on your tenses. Try to keep everything in "literary present." You fall into the passive voice here with "war has." I am thinking that the first part of the sentence isn't meshing with the second. I'd probably drop it altogether and say something like this instead: War quickly robs the young soldiers of their youthful thoughts and innocence.
The German propaganda machine also spared no effort in dehumanizing the enemy.
"Spared" isn't working here. Sparing effort is like avoiding effort. You might spare a friend embarrassment by covering up a small fault for him. You could also spare a friend the effort of going to school to pick up missing assignments by bringing the assignments to him. You could change it to something like: The German propaganda machine effectively sought to dehumanize the enemy.
Ironically, the schoolmaster of the young boys writes to them, referring to the boys as "the Iron Youth." According to the narrator, "We are none of us more than twenty years old.
You might want to explain the irony a bit here. I am not sure how to include it, but I feel like there needs to be a more explicit contrast drawn between the then and now and how the boys no longer see themselves as youth. Maybe: The soldiers find it ironic when their schoolmaster extols them as "the Iron Youth" in a letter; war matures the boys. Even though none of the boys is older than twenty, they question their youth: "But young? Youth? That is long ago. We are old folk" (p. 360). Okay, I am not crazy about that construction either, but I trust your ability to rework it just a tad.
Once war ensued in 1914, this formerly benign distaste for foreign opposition became overt and necessary for success as new recruits were needed at the front lines of a never-ending war of attrition.
Awesome sentence. "Were needed" slips into the passive voice. New recruits replaced dead soldiers? New recruits marched to the front lines?
Without such a fervent hatred for the other side, little cause for such violence would exist.
Again, great sentence. Take out "would" and just use "exists."
The Great War of 1914-1918 was traumatic for both the nations of Europe and the people who were involved within it.
That passive voice is so tricky! Instead of "was traumatic," you could write "traumatized." "Who were involved within it" gets wordy and relies on the passive voice. Just say "the people involved."
According to Remarque's "All Quiet on the Western Front," the soldiers of the Great War were young boys whose only crime was to trust an institution that would send them to die for vague ideals such as glory and patriotism.
Again, the passive voice sneaks in. Try something like: In All Quiet on the Western Front
, Remarque writes of the soldiers of the Great War as innocent young boys trusting in an institution that sends them to die for vague ideals such as glory and patriotism. That brings up another point ... italicize the title of the book instead of putting it in quotes for your final paper.
At the same time, their side and opposing factions pumped out propaganda to suggest that these young soldiers were not human in any right, but instead savage beasts to be destroyed for the good of the world.
Because it is a literary paper, I think it best to stick to the literary present even here. Try to omit "were" and "to be." Hmmmmm ... Opposing sides pump out propaganda suggesting the inhumanity of the enemy and portraying the opposition as savage beasts in need of destruction for the good of the world.
This propaganda displayed not only the detriments of nationalistic competition, but also served to perpetuate the war by inciting hatred between groups of people that did not naturally exist.
Tense and voice. This propaganda displays not only the detriments of nationalistic competition, but also serves to perpetuate the war by inciting hatred not naturally existing between groups of people . I switched around the last part of the sentence because it is the hate that doesn't exist naturally not the groups of people that don't exist naturally (that would be Brave New World
The effect was a bitter and prolonged conflict that incited deep mutual hatreds between nations of the world that exist to this day.
Tense and voice. The propaganda effected a bitter and prolonged conflict inciting deep mutual hatred between nations of the world existing to this day.
I just reread what I wrote and realized that a lot of the corrections are still falling into past tense. Check your tenses when you rewrite to ensure things make sense and flow. Best of luck!