The original US Constitution was very coy about slavery - so much so that the word ‘slave’ never appears. Instead, we get “Importation of Persons” and “3/5s of other persons” and similar work-arounds. This suggests that a significant number of the Framers didn’t like the idea of explicitly acknowledging slavery and that even those who were slave-owners sensed that it wasn’t popular to be explicit about it. The Article that prevented Congress from outlawing the slave trade for decades implies that certain Framers were concerned that the US might soon have a majority who did want to outlaw slave trade. But no matter how coy they were, the Constitution plainly allowed slavery - and over the next 80 years, slavery advocates got over their reluctance to talk openly about slavery, making a crime and sin into a matter of pride for a whole region of the country. Lincoln’s Proclamation takes the slavery advocates at their word: it’s perfectly legal to confiscate the property of rebels: if slaves are property then Lincoln had the legal right to free them.