"If the cause is just, which yours is not."
(Setting aside the arguement on whether the South had such a "just cause" or not at the moment)
You are yet to establish that secession requires a "just cause" to be Constitutional, or even Biblical. You are also yet to establish what "just cause" the North had in invading. The South made no attempts to assimilate the North, why did they (the North) attack it (the South)?
Trust me, the Confederates that fought and died were fighting for a cause. The Yankees recognized this durring battles. Many wrote home about it (as has been discussed earlier in this thread)
And no: "slavery" was not on their "to die for" list. If it were, we'd still have that sentiment today. (Another fact you ignore)
Lincoln put down an armed insurrection, just like Washington did when he put down the Whiskey rebellion.