Final answer:
In the two decades following Civil War commentary, the portions of the United States most affected by immigration were the industrial cities of the Northeast and Midwest, where immigrants primarily remained in urban centers, significantly impacting the demographic and economic landscape.
Step-by-step explanation:
In the two decades following Civil War commentary, the most affected parts of the United States by immigration were the industrial cities of the Northeast and Midwest. More than 200,000 immigrants arrived yearly in cities like New York alone. These new immigrants, primarily from southern and eastern Europe, were unlike their predecessors and often lacked the financial resources to push westward. As a result, they remained in the urban centers looking for work. These urban areas saw significant changes in their structure, evolving from walking cities of colonial America to the industrial hubs driven by the influx of immigrants seeking employment and a new life, forming ethnic enclaves and changing the demography profoundly.
It was not only the urban centers of the East and Midwest that were affected. Anti-immigrant prejudice, illustrated by the scapegoating of an Irish immigrant for the Great Chicago Fire, was widespread, demonstrating how immigration and the responses to it shaped American cities and attitudes. Thus, the effects of immigration were most prominently felt in the urban areas of the Northeast and Midwest, radically altering these cities' demographic, economic, and cultural landscapes during this period.