The "tyranny" was "Slave Power," as it was decried in nearly every newspaper in the North and West United States. The same tyranny that sought--but failed--to wrest Kansas in the wake of the Kansas-Nebraska Act. The same Tyranny that sought, in the wake of the Dred Scott decision, to force a repeal of every law limiting slavery anywhere in the U.S., and allow slave ownership to flourish everywhere--which the vast majority of the people did NOT want.
It was that same tyranny, represented by the Democrat Party then as it is today, that put forth an unpopular but politically powerful lobby to try to tear out the very roots of this nation's founding.
Those tyrants were foiled by the election of Abraham Lincoln, who although he promised them their rights local to those few states they controlled wouldn't be abridged, nevertheless swore just as strongly that slavery would not expand one more inch into the new territories. Because of that fact, they rebelled.
PURELY, SOLELY, ONLY FOR THE CONTINUATION AND PROTECTION OF SLAVERY AND FOR NO OTHER REASON.
Revisionist historians like the fanciful dreamers here on FR may rant all they want, but that is immutable FACT.