Lincoln needed foreign countries to support the Union and not support the Confederacy.
He knew that England would not support the Confederacy if the Confederacy allowed slavery. This is because England had already outlawed slavery, and they lost many of their slaves that came to the new world.
England was leaning toward supporting the Confederacy, due to the fact that England traded with them and since their aristocracy was more like the Confederacy’s government, and less like the Union’s republic .
But England could not support the Confederacy if the Confederacy supported slavery.
Lincoln understood that foreign support for the Confederates would decrease the Union’s chance of victory, so if the Civil War were focused on slavery, then England would not support the Confederacy.