“The tariff had been opposed by many Southern legislators, which is why it passed so easily once their states seceded. “
Either they did or they didn’t. If they did then Lincoln committed acts of war against a sovereign nation. That statement, alone, says someone is acknowledging that the southern States did secede and votes were held in the US congress with such acknowledgment.
If that were true, then didn't the Confederacy commit the first act of war by firing on Sumter?