Lincoln’s sworn duty was to uphold The Constitution, not to ‘preserve the union’
The Constitution ALLOWS for States to secede
It does not say you must go to war to stop them
The war was about states’ rights, and slavery was the vehicle involved. That is all I am saying, it could have easily been some other reason the states wanted to secede, but they do have that right.
The Constitution allows states to secede, by amendment, by federal law, or even by successful supreme court case.
It does not allow states to secede unilateraly. Rather, Article 3 of the constitution requires states to resolve controveries between them and the federal government at the Supreme Court.
Can you point me to that clause in the constitution?
It does not say you must go to war to stop them
See Article II, Section 3 of the US Constitution. Also see Section 2 of the Militia Act of 1792.