What helped Lincoln decide to address the issue of slavery in the south ?

C

Explanation:

He wanted to show the world that this war was about slavery and since the world had pretty much decided slavery was wrong already this would mean that no one could help the south. This was important because England relied on the South for much of its cotton and the Union was worried that this means that England would support them. If the Union had to fight against England as well they would surely lose.