Although the U.S. Civil War had multiple and complex causes, the question of slavery was a very important factor.
As slavery was part of most civilizations on all continents (especially before the last couple of centuries), and in many cases the slave class had a different ethnic makeup than the privileged class, it occurred to me that I know about many slave revolts from different eras and locations, but all of them involve the slave class fighting against the privileged class.
In the US Civil War the overwhelming majority of the fighters of the anti-slavery side were not members of the slave class. Was this unique in history? I never heard about an army of Ottomans or Mongols or Romans or Almohads etc. to fight for the freedom of the very ethnic groups who served them (or at least people from the same group as them) as slaves.
Members of a privileged group fighting (as in risking their lives in battle, not just making speeches) for the freedom of a group they are not part of. Was this thing unique for the US civil War? If not, was it the first case?
I wouldn't count a few sympathizers who join a slave revolt despite being of higher social standing. For an example to count, the anti-slavery fight has to be started and led mainly by people who are (or could be if they wanted) members of the slave-owning class.