Anybody who has read Lincolns Inaugural Address or who has read the Corwin Amendment knows slavery was not threatened in the US. Its preservation could have been ensured permanently with the approval of Texas and other Southern states.
The original seven Confederate states had seceded before the Corwin Amendment was offered and before Lincoln's inauguration, and having adopted a constitution that protected slavery to a far greater extent than the U.S. Constitution did.
The southern slave owners thought differently. And acted accordingly.
Slavery was no more threatened than the 2A today.