How did the emancipation proclamation open the door for for black soldiers

In the Summer of 1862, Lincoln realized that emancipation was required for political and military necessity. Many different factors led to this reason, with lack of military success, hope that emancipated slaves might help meet the army’s growing manpower needs, changing northern public opinion and the calculation that making slavery a target of war effort would counteract sentiment in Britain for recognition of the Confederacy.  So at the advice of Secretary of State William H. Seward Lincoln waited until a Union victory to announce it and five days after McClellan's army forced Lee to retreat at Battle of Antietam. Lincoln issued the Preliminary Emancipation Proclamation which warned the South if it did not surrender by the end of 1862 he would abolish slavery. The reaction in the North was not good as they would lose a lot of their legislature position and the Democratic Party used this as opposition propaganda. On January 1, 1863, Lincoln went to his study to sign the Emancipation Proclamation. It did not liberate all slaves and only a few. The document had exempted areas under Union control (where the war in effect, had already ended) and did not apply to loyal border slave states. In addition parts of the Confederacy occupied by Union soldiers were also exempt such as Tennessee, and parts of Virginia and Louisiana. But the remaining majority of the South’s slaves more than 3 million men, women and children were declared free but since most of these slaves were still behind Confederate lines they would need to wait until Union liberation. But it encouraged south slaves to run toward free states and ended any possible negotiation with the Confederacy. It would cause a large number of black slaves to join the Unions side of the war. This document changed the course of the Civil War which was started to preserve the prewar Union now was meant to create a new definition of freedom. A new system of labor, politics and race relations would have to replace the shattered institutions of slavery.

Source: College US History Class I took a year ago.