See http://www.freerepublic.com/forum/a39fce5ba5cc1.htm
"the slaveholders would never had dared, in the face of the world, to attempt to overthrow a government that gave freedom to all, for the sake of establishing in its place one that should make slaves of those who, by the existing constitution, were free." (pp.2-3)
By defending their own freedom, rather than slavery, Southerners gained a great psychological and moral advantage that carried them through four years of war. In agreeing that the Constitution protected slavery, and by proposing compromises in 1861 to prevent succession, Sumner and others only weakened the moral position of the North. Against the Northern politicians, generally, Spooner charged that "upon your heads, more even, if possible, than upon the slaveholders themselves, (who have acted only in accordance with their associations, interests, and avowed principles as slaveholders) rests the blood of this horrible, unnecessary, and therefore guilty, war." (Letter to Sumner, p.3)
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Spooner did not speak against the US military for taking action to end slavery, he spoke against the North's tacit admission that slavery was Constitutional and merely needed reforming while prosecuting the war over the issue of seccession.