Final answer:
Walter's fight for the Nazis is ironic because it represents support for the hate-filled and nationalistic Nazi ideology while striving to overcome Germany's national humiliation from the Versailles Treaty without understanding the regime's true inhumanity.
Step-by-step explanation:
The irony in Walter fighting for the Nazis lies in the fact that the Nazi ideology, which promoted the superiority of the 'Aryan race', was exceptionally nationalistic and anti-democratic, and it capitalized on the deeply felt national humiliation of the Versailles Treaty. Those who fought for the Nazis, therefore, supported not only the reversal of the post-World War I conditions imposed by the Versailles Treaty but also the broader, hateful ideology that targeted Jews, communists, and other groups as scapegoats for Germany's problems.
It is ironic because Walter, like many others, was likely seduced by the promises of national restoration without fully comprehending the inhumanity of the regime's policies or the eventual cost of its actions.