Murray Rothbard at his best:

The basic root of the controversy over slavery to secession, in my opinion, was the aggressive, expansionist aims of the Southern "slavocracy." Very few Northerners proposed to abolish slavery in the Southern states by aggressive war; the objection – and certainly a proper one – was to the attempt of the Southern slavocracy to extend the slave system to the Western territories. The apologia that the Southerners feared that eventually they might be outnumbered and that federal abolition might ensue is no excuse; it is the age-old alibi for "preventive war." Not only did the expansionist aim of the slavocracy to protect slavery by federal fiat in the territories as "property" aim to foist the immoral system of slavery on Western territories; it even violated the principles of states' rights to which the South was supposedly devoted – and which would logically have led to a "popular sovereignty" doctrine.

Murray Rothbard
http://www.lewrockwell.com/rothbard/rothbard175.html