It was caused by the secession of states that produced a great sum of money which flowed through the pockets of powerful Washington DC and New York interests.
When Cuba wanted independence, there were few to none opposed, because it was not making the Northern elite a great deal of money. Same with the Philippines, and Puerto Rico, of course, has an open invitation to leave at any time they so desire.
Again, none of them have major amounts of production which would cause great losses to the power blocks currently in control of our government.
But the South did. The South produced 73% of all the taxes flowing into Washington DC in 1860. It produced the vast bulk of American trade with Europe in 1860, with virtually all of that money flowing through the pockets of New York shippers, bankers, insurance agents, distributors and so forth.
The South was a huge economic cash cow for the Northern powerful.
So to be clear, it wasn't secession that cause the war. It was the secession of states producing great sums of money for the powerful interests in the North, that caused the war.
In other words, it was about who was going to control that monetary output of the southern States.
Beyond that, the Declaration of Independence asserts the right of all states to leave whenever they come to believe a government no longer serves their interests, and so therefore the Southern states had a moral and legal right to leave the Union when they came to believe it no longer served their interests.
The wealthy New York merchants and businessmen were not opposed to slavery for the most part.
In the 1850’s America had a one party Democrat controlled government. The Democrats were trying to create a slave based economy in the midwest and the west. Most Americans did not want a backward, slave based economy.
The idea that the slave based south was the primary source of American wealth is simply not true. The manufacturing and innovation in the north made it far more prosperous.
During the Civil War the south was unable to adequately supply their army. The South was mostly poor, backward and uneducated. There were a few wealthy plantation owners but there was not the level of prosperity that was in the north and the midwest.
Most southerners opposed secession. It was pushed through by a small group. By the end of 1864 there was no support left in the south for the Confederate government and soldiers were deserting in large numbers.
Lincoln and most Americans would never have gone to war just to end slavery. The Republicans did not want to abolish slavery. They wanted to prevent a slave based economy in the Midwest and west. And they wanted to preserve the union.