You are most likely referring to the assassinations of Martin Luther King Jr and Malcolm X. The impacts of both of their assassinations was that the Civil Rights movements actually grew stronger, with more African Americans and white Americans coming together to fight for equality on the streets, in Presidential elections, and through activism in Churches. Both MLK and Malcolm X were good men who fought for the rights of African Americans, and they will forever be remembered for their lasting impression on this nation, not only by other African Americans, but by all Americans.
Justinian I failed to regain all of the former provinces of the old Roman Empire really from a combination of all these reasons, but the main one was that he made military blunders.
