I define unity almost the same as a team.  Unity is a group of people working together to achieve a common goal and increasing their strength, drive, and knowledge through sharing.  Diversity is the focusing on differences in people causing balkanization and failure to work towards a common goal.  Diversity is what is stressed by a weaker force conquering a stronger group - one segment at a time.  True leaders build teams and unity, they do not focus on the differences in petty things such as sex, race, origin etc.  Dividers stress differences.