Here are some different arguments you could make for this question:
1) Equal legal rights for African-Americans- The Civil Rights movement and the responding laws, like the Civil Rights Act of 1964 and the Voting Rights Act of 1965, were two huge steps forward in making America the "more perfect union" it is today. This is because a country is supposed to be a collection of people that are able to have equal rights in the eyes of the law.
2) Women's rights- The Women's rights movements of the early 20th century and 1960s-1970's resulted in women getting the right to vote and gaining legal rights they never had before. This helped towards reaching a more perfect union because it allowed for men and women to have the same legal rights.
