What made Lincoln decide that emancipation was a military necessity?

The consecutive losses of the Union in battlefields made Lincoln decide that emancipation was a military necessity.

Explanation:

President Lincoln was well aware of the military might of the southerners and was doubtful of their victory in the civil war after consecutive losses faced by the Union. He also knew that if the Union succeeds in emancipating the slaves in the south, they would come and join the war on the side of the Union and that would considerably increase the strength of the forces of the Union.