ironhead
anarcho-capitalist
Lincoln's intent - whether you agree with it or not - was to prevent the south from seceding, thus preserving the union. (Another aspect of Lincoln's strong federalist beliefs.) By his definition - which was later upheld by both SCOTUS and Congress - the declaration to secede was insurrection.
Anyway, you call his war "unconstitutional". You use the term out of ignorance of the facts. Ditto people calling Vietnam, Korea, and the current Iraq wars "unconstitutional"
I invite you to study the Prize Cases from the Civil War, in which SCOTUS made its first and most encompassing decision regarding the power of the president as Commander in Chief. No challenge since has made a significant change to the Prize Cases decision. It is why Korea and Vietnam were allowed to take place despite there being no War Powers Act. It is also why all sitting presidents since 1973 (of both parties) have claimed at one time or another that the WPA itself claims unconstitutional authority by congress over presidential powers as CinC.
http://www.answers.com/topic/prize-cases
http://caselaw.lp.findlaw.com/scripts/getcase.pl?navby=case&court=us&vol=67&invol=635
Insurrection might have been interpreted that way, but I can't agree with that, considering that the South was not trying to overthrow the government of the North, it was trying to secede from it. The Constitution was ratified by the original 13 colonies, in other words it was agreed upon at that time, but each state reserves the right to secede as a sovereign entity. SCOTUS may not have interpreted it that way, but that's not the only time I've disagreed with them (Dred Scott, Plessy vs. Ferguson, Kelo, etc.).
Mind you, I strongly feel the North should have purchased all of the slaves in the South and set them free in the North, ending slavery as other nations have done.