Final answer:
The Twenty-Sixth Amendment expanded citizenship in the U.S. by lowering the voting age from 21 to 18, ensuring that younger adults have the right to vote, especially after the Vietnam War highlighted the importance of this demographic being able to influence policies that may affect them.
Step-by-step explanation:
The Twenty-Sixth Amendment significantly affected the concept of citizenship in the United States by extending the right to vote to younger adults. Specifically, it lowered the voting age from 21 to 18 years old. This change was prompted by the involvement of the United States in the Vietnam War, where the criticism was that young men, eligible to be drafted for military service at 18, should also have the right to vote on the policies that directly affected their lives. Before this amendment, the voting age had not been uniform across all states, and this constitutional change created a consistent national standard for suffrage.
Historically, the expansion of suffrage in the United States has been a gradual process. The 15th Amendment established voting rights regardless of race, the 19th Amendment provided women the right to vote, and the Indian Citizenship Act of 1924 and the McCarran-Walter Act of 1952 extended suffrage to Native Americans and Asian Americans, respectively. Poll taxes were abolished by the 24th Amendment, contributing to the elimination of economic barriers to voting.