The North, already threatened several times by a secession that had never taken place, did not accept the idea that the grumblers would seriously contemplate a separation. It was dominated by the radical party who, being for centralization, intended, in the name of the greatness of a common fatherland, to annihilate all local opposition, and who was soon to gain the Abolitionist Party as an ally.
The Abolitionists, by the most noble motives, carried the same wishes as the radicals. For a long time, they had thought using only moral means for the emancipation of the slaves, but when they came to understand how greatly their principles would gain from the application of the new rules, they entered eagerly into the fight between parties.
The abolitionists were those who, through moving, eloquent books had revealed the shame and miseries of slavery. They had easily succeeded in inspiring horror for it. From the moment they put their banner next to the one representing the interests of the North, they rallied all the generous minds who were set aflame by the perspective of the emancipation of an unfortunate race.
The problem of slavery, much easier to understand than that of the rights (be they unquestionable or not) of the states, and of their dealings with the federal government -- took rapidly priority, abroad, over all the others. From a distance, the problem of slavery was the only one perceived.