[AnalogReigns]

labeling of slavery as the casus belli of the Civil War ...

Hence, maintaining the Union was his prime motivation...

Hmmm, a direct contradiction, in the first 2 paragraphs, not a good sign.

So was the slavery the casus belli, or was maintaining the Union?

Seems clear that Lincoln's reason for invading the South was to save the Union, NOT to end slavery.

Has the USA been less constitutional, and (much) more centralized on the federal government since? Yes. Absolutely.

[Non-Sequitur]

So was the slavery the casus belli, or was maintaining the Union?

Slavery was the prime motivator for the Southern side. Maintaining the Union was the prime motivator for the Northern side.