Slavery ended throughout the western hemisphere without war, except in the case of Haiti and the United States. But the true believers claim that slavery wouldn't have ended in Brazil, Cuba, and the other western societies without the American Civil War as a prod. I have to give them credit for knowing that they ought to try to justify the slaughter, despite the apparent absence of any logical connection.
There is not a single instance where slavery was not ended through government legislation, and always over the strenuous objection of the slave owners themselves. Since the southern states were willing to rebel to protect slavery in 1860 how long do you think it would be before they would be willing to sit back and accept a forced end to their institution? 20 years? 50 years?