I can give you an example of a possible response to this assignment prompt.
One thing that could have been done differently to prevent World War II was for the Western powers, specifically the United States and the United Kingdom, to have taken a more assertive stance towards Nazi Germany's territorial expansion in the 1930s.
This could have been achieved by imposing harsher economic sanctions and military threats against Germany, which could have deterred Hitler from continuing his aggressive policies.
The US did not take this action at the time due to a combination of factors, including the legacy of World War I, isolationist sentiment among the American public, and political divisions within the US government.
The US government at the time argued that imposing harsher measures against Germany would lead to war and further destabilization of Europe. However, in hindsight, it is clear that this reasoning was flawed, as the appeasement policies of the Western powers only emboldened Hitler and allowed him to gain more power and territory without facing any significant opposition.
Do you know about appeasement? This was a fairly useless policy that the US implemented to prevent Hitler from demanding territory. Hitler went:
"I want that country"
USA went:
"Noo, you can't have that country.." (Hitler takes control of that country anyway)
"Ok fine, you can have that country, but no more okay?"
Hitler:
"I want that country."
AAAAND REPEAT
Therefore, a more assertive stance towards Nazi Germany could have prevented the outbreak of World War II, and the US government's justification for not taking this action was not justified in the long run.
~~~Harsha~~~