How so? What demonstrable danger did the secession of the Southern States present if they had been allowed to leave in peace as they requested?

Was the Confederate States trying to overthrow the Government of the United States and depose Lincoln from his position as President of that country? NO!

The south fought a DEFENSIVE war. Which by definition is not a war of aggression, which would be necessary for there to be a rebellion. Rebellion being defined as an aggresive attempt to overthrow a setting government.

So if the threat were "domestic" that threat would have had to have been a rebellion. Or Lincoln could not legally use the United States troops against American citizens. If the Citizen of the Southern States were acting within their rights under the Constitutionthen there is no rebellion. Rebellion is not a right granted under the Constitution. Revolution is another matter. Such useage of federal troops was and still is Illegal. And especially when authorized by the President of the United States against it "citizen", which Lincoln claimed that they were several times. He had to claim that they were still U.S. Citizens for them to be in rebellion. But the Constitution does not prohibit secession, therefore under the 9th and 10th amendment it was a right granted to the states.

History can't have it both ways. Either the people of South Carolina were still part of the United States, OR they were not.

In either case Lincoln was wrong in taking any hostile actions against these people, because they did not take any actions to overthrow the Government of the United States. They mearly seceeded and asked the United States to remove it troops from their territory. The United States were the aggressors starting with the occupation of Fort Sumter against presidental orders and proceeding from that point forward.

If the threat was of a "foriegn" nature then the southern states were no longer part of the United States. Again which Lincoln still insisted that they were still a part of the United States. If they were not a part of the United States then they could not have been in rebellion, because rebellion is only an internal matter. Disagreements between two foriegn powers is not Rebellion. In that case the occupation of Fort Sumter and the efforts to reinforce and blockade the harbor of another nation was a de-facto act of agression and War, without the authorization of Congress. By the way, you can not legally blockade a harbor of your own people with Military forces either, for the same reason as stated above.

Lincoln violated his oath of Office because he did not uphold the Constitution. He "threw it under the Bus", to use a popular phrase of today. And did so more than once arresting citizens and jailing them for years or deporting them without charges, and eliminating the rights of free speach and peaceful assembly even in the Northern states, under the guise of the Southern States being in rebellion.