Conversation so far:
What did George S. Boutwell do in the early civil war years?
Boutwell attended the Peace Conference of 1861 in Washington, D.C. which attempted to prevent the impending Civil War
What was George S. Boutwell's role in preventing the impending Civil War?
In the peace conference, he angrily rejected Southern proposals favoring the extension of slavery and its enforcement in northern states

Question: what was his reason for rejection?
Self-contained rewrite: what was George S. Boutwell reason for rejection of Southern proposals?