Final answer:
The Emancipation Proclamation, signed by President Lincoln on January 1, 1863, declared that all enslaved people in Confederate states were free. It was a landmark decision that redefined the course of the Civil War and made abolishing slavery a central goal of the Union's effort.
Step-by-step explanation:
President Lincoln's announcement about enslaved people being free was called the Emancipation Proclamation. The Emancipation Proclamation was a historical document signed by President Abraham Lincoln on January 1, 1863, during the American Civil War. This proclamation declared that all persons held as slaves within the rebellious states are, and henceforward shall be free. However, it primarily applied to the states in rebellion, specifically the Confederate states, and did not cover the border states or regions under Union control. Hence, its immediate effect on slavery was limited. Despite this, the Emancipation Proclamation was a significant step toward the abolition of slavery and shifted the goal of the Civil War towards ending this institution.
After a preliminary version released following the Union victory at Antietam, the final Proclamation emphasized that liberation of slaves in rebel states was a matter of military necessity and an act in pursuit of preserving the Union. The Emancipation Proclamation also set the stage for future attempts to completely abolish slavery, which eventually happened with the ratification of the 13th Amendment.