The same could be said of Robert Lee, Jefferson Davis, and just about every other confederate leader.
How?
None of them invaded the North until it became apparant that a victory needed to be won decisively on Northern Soil in order to gain foreign recognition.
Lee did not violate his oath when he resigned from the army.
Neither did Davis or any of the others.
The Federal Government had NO right, and still do not, to compel states by force of arms to stay in the Union.
The Power and Authority to Secede from the Union, for ALL states is contained in the 10th Amendment to the Constitution which reads as follows.
The powers not delegated to the United States by the Constitution, nor prohibited by it to the States, are reserved to the States respectively, or to the people.
Nowhere in that document does it state that the Federal Government has the authority or the power to compel states to remain in the Union. As such, they do not have the power, but the States involved had every right Under the Constitution, to dissolve that union.
Now, I will change my mind if I can be shown proof positive that the Constitution expressly gives the authority to the Federal Government, to prevent states from leaving the Union.
Until then, my position remains firm and I believe, on solid Constitutional grounds.
The power to approve a change in the status of a state is an power delegated to Congress, and by implication that would include leaving altogether. The power to act in a unilateral manner where the interests of other states are impacted is a power explicitly denied to the states in a number of ways. By implication that should include the ultimate impact, secession.