Final answer:
The hardest post-World War II challenge for the U.S. was demobilization and reintegration of veterans into civilian life, alongside internal and international issues including women's roles, New Deal policy debates, establishing international order, and addressing civil rights.
Step-by-step explanation:
The hardest challenge facing the United States after World War II was the issue of demobilization and the reintegration of veterans into civilian life. Although the U.S. emerged from the war economically strengthened, with advancements in technology and military supremacy, the transition to peacetime was not without its difficulties.
These included the return to civilian life of millions of servicemen, the question of women's roles post-war, and the debate over the continuation of New Deal programs. Additionally, the U.S. was tasked with taking a leading role in the creation of the post-World War II international order, aimed at maintaining global peace and economic prosperity, while also addressing internal concerns such as civil rights and the new dynamics of a peacetime economy.