What opportunity did Lincoln see shortly after the battle of Antietam?

To rally Northerners for a common cause and hurt the South

Lincln decided to...

...Emancipate, or free the slaves with the Emancipation Proclamation

What was the catch?

Only slaves in states that were part of the Confederacy would be considered to be free

Border States

Several States that still allowed slavery, called border states, had remained in the Union (Delaware, Maryland, West Virginia, Kentucky, and Missouri). Slaveholder in these states kept their slaves even after the Emancipation Proclamation

Silly southerners.....

The South was furious that the President had ended slavery, but it had little actual effect in the Confederacy

In the North,

...it made the abolition of slavery a firm goal of the Union cause.

The Proclamation joined the original goal of...

...preserving the Union