Pius IX and the CSA

I need a dedicate-post on Pius IX's correspondence with the American Confederacy (of slaveholding rebels).

This is because, for some reason, Catholics think wise to cite Pius IX. He was, in my (limited) research, the most unwise bishop Rome has seen, since at least the Council of Trent. The bill against him, in history, is long: from the self-sabotage of Maximilian's restoration of the Habsburg imperium in Mexico, to the Mortara case, to his ineffectual attempts in holding his estates and then whining about it as "prisoner of the Vatican". Papal Infallibility alone has driven a wedge between us and the Orthodox. Then there's the deeply-problematic doctrine of the Immaculate Conception.

As for the CSA: Pius' apologists would cast his intent to reach peace and to emancipate (in stages) the South's slaves, as was perhaps the hope in Union slavestates like Maryland. Judah Benjamin and general Longstreet may have used this in their own manumission plans. Rather than imposing penance, this Pope would accept repentance from within. He was not this guy.

If this was this Pope's intent - it didn't help. Why didn't it help? I think, any Christian recognition of the rebel states would require those states' politicians undergo metanoia: in this context, to repudiate their own statements of secession. That's what the Thirteenth Amendment would be. Was that going to happen by negotiation?