Why do you think he calls the act a “military necessity” in the last section

Referring to the emancipation proclamation, i believe the reasons are: 

- The Union would gain people and the Confederacy would lose people.  (because the millitary draft would very likely to favor the union due to the amount of African American that support them)

- The change would help the Union win.
(More soldiers mean that the union army would be able to utilize more millitary strategies that require large amount of manpower)

Lol that is s funny : )