Why did lincoln need a military victory before he could announce the emancipation of slaves in southern states?

He didn't wanted the emancipation proclamation to look like an act of desperation.

Well the reason was because the South had seceded from the Union and became the Confederacy. They created their own government and ideas of how to run their government. The original Emancipation Proclamation was signed in 1862 which gave the free African Americans in the North the ability to fight in the Civil war which created the 54th Massachusetts infantry which was an all black infantry except for the commanding officer Robert Shaw.