The right of revolution is an inherent one. When people are oppressed by their government, it is a natural right they enjoy to relieve themselves of oppression, if they are strong enough, whether by withdrawal from it, or by overthrowing it and substituting a government more acceptable.
In his later reminiscences, Ulysses S. Grant roundly condemned the Mexican War in which he had served, and even saw the Civil War as a sort of karmic retribution for America's sins against its southern neighbor: "Generally the officers of the army were indifferent whether the annexation [of Texas] was consummated or not; but not so all of them. For myself, I was bitterly opposed to the measure, and to this day regard the war, which resulted, as one of the most unjust ever waged by a stronger against a weaker nation. It was an instance of a republic following the bad example of European monarchies, in not considering justice in their desire to acquire additional territory."
"The Southern rebellion was largely the outgrowth of the Mexican war. Nations, like individuals, are punished for their transgressions. We got our punishment in the most sanguinary and expensive war of modern times."