It probably would have been easier to resist calling the secessionists "the enemy" if, on the way out the door, they didn't try multiple times to assimilate President-elect Lincoln, and if they didn't lay a siege on U.S. island Fort Moultrie and attack Fort Sumter.
On Nov 20, 1860, Lincoln even said that his administration would allow states to control their own affairs. And he traveled then neutral, but pro-slavery Maryland to tell them so. But the war mongering Dims tried multiple times to kill him. Later on March 4, 1861, on his inauguration day Lincoln again says that states will be allowed to handle their own affairs.
On March 8, 1861, Robert Hunter, the new Confederate Sec of Foreign Affairs, proposed that the U.S. could avoid war if they treated the Confederacy as a "belligerent power", not as a new separate nation parting ways with the U.S. It'd be analogous to what Israel has had to put up with since 1948 of having organized groups within their border always at war with Israel, always claiming they need Israel to meet their demands to have peace, but never being satisfied. Calling them "secessionists" would be about as truthful as calling Biden "brilliant"
These events by the 19th century war-mongering Dims were BEFORE they attacked Fort Sumter in April 1861.
Now you know what it was easy to see them as "the enemy".
And motivated by the highly greedy Radical Republicans who pulled Lincoln through an election in which Southern States did NOT participate. This is a major point about the influence of these political manipulators of the abolishing movement (the Puritan liberals of New England— ironically powered by money their own constituencies made in the slave trade shipping business, based out of Boston, MA and financed through NYC and Philadelphia financial houses. All of which were pro-slavery for the massive profits derived).