Answer: To free most slaves in America, namely those in the rebelling states, thus weakening said states.
Step-by-step explanation:
The Emancipation Proclamation was an Executive Order by President Lincoln during the American Civil War by which he freed all enslaved people in the rebelling Confederate states only leaving those in few Union states that still practiced slavery.
He did this to weaken the Confederate states because the proclamation would ensure that the enslaved people in rebel territories would go against their masters knowing that the Union would free them. It worked because many escaped the South and joined the Union army while thousands of free blacks joined the army to liberate their people.
The proclamation also led to reduced support for the Confederacy from European nations who did not want to be seen as agitating for slavery.