The union needed the help of African-American soldiers if they were to win the war. By releasing the emancipation proclamation, Abraham Lincoln declared that all slaves in Confederate territories would be free. Since they were now theoretically free, if the Union army managed to reach areas where there was a lot of former slaves, they would now be able to join the army and help free other slaves. They were no longer property that belonged to the slave owners, but free people who would fight slavery in the army.