Final answer:
The Union's realization that it would have to destroy slavery in order to defeat the Confederacy came gradually as the war progressed and evolved. The Emancipation Proclamation was a crucial step in making this clear. By freeing slaves, the Union aimed to weaken the South and gain support from anti-slavery nations.
Step-by-step explanation:
The realization that the Union would have to destroy slavery in order to defeat the Confederacy came gradually as the war progressed and evolved. While ending slavery was not the Union's primary goal from the outset, the Emancipation Proclamation, issued in September 1862, made it clear that emancipation was a crucial part of the Union cause. By freeing slaves, the Union aimed to weaken the South's ability to fight, recruit African Americans as soldiers, and gain support from anti-slavery nations like Britain.