We saw that many people in this country had a one-sided idea of who was kept free by our flag, as blacks provided the highest percentage of casualties. what does this mean?

It meant that even in America, racial discrimination or racism was being practiced and people were doing nothing about it. The black people were not considered part of the country because they were not given the equal freedom of the white people. Many of them were also killed and tortured. It's about time that the people have to tell the government about its wrongdoing and stand-up against it.

John Kerry was stating this in order to let the United States know that they should choose to have "peace" with the Southeast Asian countries like Laos, Cambodia and Vietnam rather than invading them.

Explanation:

The question above is related to John Kerry's testimony before the Senate Panel in 1971. He was proposing that the United States of America should end the war. He acted as a representative of the Vietnamese veterans who thought that America betrayed them by telling them to invade Vietnam for the country's sake.

However, they came back to the country, not proud because people hated them for what they did in Southeast Asia. They fully understood how Vietnamese felt when they resisted to the colonization of America.