They are all wrong.
The Civil War did not start, as many thought, over the secession of South Carolina and her sister states in the Confderacy. Let us examine the sequence of events leading up to hostilities.
Abraham Lincoln (R-IL) was known as an abolitionist. In those days, racism was still alive and well, even among the northerners who wanted to abolish slavery. Lincoln can be shown by his own words to have been "racist" in the modern sense: He believed that whites were inherently superior to blacks, in terms of civilization, culture, and intelligence. What he also believed, however, was that these were not the relevant characteristics in determining the dignity of a human being. Whether or not a black man is less intelligent than a white man, and whether or not a white man can jump higher than a black man, the crux of the matter lies in the fact that they are equally deserving of dignity, because they are equally human. Among the abolitionists, and even the racists of the north, this was not in doubt.
Abolitionists were against the extension of the "peculiar institution" of slavery into other parts of the Union. Because of the expansion of American territory after the Louisiana Purchase and the Mexican-American War, there was much land to be settled. And because of the productivity gains enabled by Eli Whitney's ctton "gin", slave labor was again profitable. When North and South came to a head over the extension of slavery to the new territories, compromises had to be made. There was the Missouri compromise, the Compromise of 1850, and finally, the Kansas-Nebraska Act. Little by little, abolitionists were making headway.
Because Lincoln was an abolitionist, the slave-holding South felt threatened by his election to the White House. South Carolina immediately exercised what it perceived to be its right, and seceded. A more detailed timeline can be found here. Essentially, between the secession of South Carolina on 20 December 1860, and the inauguration of Lincoln on 4 March 1861, five states had seceded, the Confederacy was established, and a Union vessel had been fired upon. What the timeline doesn't point out, but which some others do, is that in the meantime, several Federal forts were fired upon.
On 12 April 1861, the Attack on Fort Sumter, a Federal property, had begun. The United States had been attacked. The next day, General Anderson surrendered Fort Sumter. The War had begun.
The rest of the War is detail. The issue of slavery was again brought up in 1863 with the Emancipation Proclamation on 1 January of that year. In his famous letter to Horace Greeley of 22 August 1862, Lincoln wrote:
My paramount object in this struggle is to save the Union, and is not either to save or to destroy slavery. If I could save the Union without freeing any slave I would do it, and if I could save it by freeing all the slaves I would do it; and if I could save it by freeing some and leaving others alone I would also do that. What I do about slavery, and the colored race, I do because I believe it helps to save the Union; and what I forbear, I forbear because I do not believe it would help to save the Union. I shall do less whenever I shall believe what I am doing hurts the cause, and I shall do more whenever I shall believe doing more will help the cause. I shall try to correct errors when shown to be errors; and I shall adopt new views so fast as they shall appear to be true views.
I have here stated my purpose according to my view of official duty; and I intend no modification of my oft-expressed personal wish that all men everywhere could be free.
Understanding his role, as President, to defend the Constitution, whose first aim, in its very Preamble, is stated to be "to form a more perfect Union", he was obligated not only to respond to the secession, but also, importantly, to respond to the military aggression from the Confederacy. Had secession been the only issue, Lincoln's task would perhaps have been diplomatic in nature. Because of the military hostilities from South Carolina, however, a military response was called for.
The echoes are important in our time, but primarily I want to remind Chinese hypernationalists of the real history of the American Civil War. The secession had been of States that willingly joined the Union in the first place; and the military action was not undertaken until the Confederacy made the first move. Thus, the American Civil War is a completely improper model for China-Taiwan relations.
Moreover, a review of Taiwanese history will show that, not only did Taiwan never consent to becoming part of China, but that it was the subject of Dutch and Spanish colonization, and had no significant contact with China until Koxinga's flight of 1664. As the Ming Dynasty was by then defunct, Taiwan did not become part of the Celestial Empire until the military conquest by the Qing Dynasty in 1683. In fact, Taiwan wasn't even a province until 1887, and was signed away to Japan in the Treaty of Shimonoseki of 1895.
I hope this will clear things up a little. Chinese hypernationalists have been and would continue to be gravely mistaken in bringing up an analogy with the American Civil War, because the background is completely different. If "re-unification" is the goal, perhaps enticing the people, who have by now become used to political freedom, must be the means. And military force cannot be used unless the Taiwanese were to fire on, say, missile silos in Fujian.
[Cross-posted at Between Worlds and Naruwan Formosa]