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

The reason why Lincoln needed a military victory was to prove that his government had the power to enforce this law.  Without a military victory, it would not be applied to the states where slavery is being practiced making useless.  Secretary of State William Seward also advised the President that it would be best to issue the Proclamation after a military victory.  When the Union army drove back the Confederate forces commanded by General Robert E. Lee at the Battle of Antietam, it was the victory that Lincoln needed to issue the Emancipation Proclamation.