Why do think lincoln calls the act a "military necessity" in the last section ?

The Union would gain people and the Confederacy would lose people. The change would help the Union win. Hope it helps

the pogroms became known as kristallnacht, the "night of shattered glass," for the shattered glass from the store windows that littered the streets. the morning after the pogroms 30,000 german jewish men were arrested for the "crime" of being jewish and sent to concentration camps, where hundreds of them perished.