The Geneva conventions were signed after the Civil war, and of course didn’t apply. Condemning Sherman for violations of that is rather like condemning Lee for violating the 13th Amendment in his possession of slaves.
Considering that rebellion was treason, I am amazed how many rebels survived. How merciful the victorious north was!
I didn’t day it did. But the Geneva Convention made an effort to “civilize” warfare and such actions by Sherman would have clearly been “uncivilized” according to the consensus of most people at the time the convention was signed.