In March 1861, after he was inaugurated as the 16th President of the United States, four more followed. The secessionists claimed that according to the Constitution every state had the right to leave the Union. Lincoln claimed that they did not have that right. Physically the states cannot separate.
Secession is unlawful.
A government that allows secession will disintegrate into anarchy. That Americans are not enemies, but friends. Secession would destroy the world's only existing democracy, and prove for all time, to future Americans and to the world, that a government of the people cannot survive. Lincoln may have thought the fifth point was the most important. If you traveled the earth in 1860, and visited every continent and every nation, you would have found many examples of monarchies, dictatorships, and other examples of authoritarian rule.