How did the Emancipation Proclamation open the door for black soldiers?

For approximately the first year and a half of the Civil War, Lincoln and his administration attempted to put down the rebels without disrupting the social order of the South—in other words, without outlawing slavery in the states where it already existed. (This fact has been used by people on both sides of the issue to attack Lincoln as a hypocrite, or to assert that the War was not really about slavery and that "state's rights" in the abstract were the cause of the conflict. Neither of these assertions is true.) By September of 1862, however, several things had changed the Union administration's attitude about how the war should be prosecuted. One was the lobbying efforts of Frederick Douglass and other abolitionists, which had convinced Lincoln that, for moral reasons, slavery could not be allowed to continue. Lincoln had always been anti-slavery, but he had imagined that the "peculiar institution" would die a natural death and that it would be counterproductive to attempt abolition through force. Another factor was that some enslaved people were already essentially freeing themselves, fleeing by the thousands across the Union lines. A third factor was that the war was, admittedly, not going well for the Union in the eastern theater, especially with the losses in the Seven Days' Battles outside Richmond in June of 1862 and at Second Bull Run in August. By declaring the liberation of the enslaved people, Lincoln knew that a huge number of African American men would be added to the armed forces of the Union. Before he issued the preliminary Proclamation after the Battle of Antietam in September, Union commanders had had mixed views on how to deal with African Americans who offered their services to the Union cause. No official position had been declared by the government about their legal status, though they had already in effect liberated themselves. The Emancipation Proclamation made it official that the enslaved people were now legally free and thus could be recruited to serve in the armed forces against the rebellion. It also ensured that the war was now directly serving the cause of abolition and basically dashed any possibility that European powers would intervene on the side of the Confederates.