This has been a tiresomely recurrent argument on Conservative boards, so I'll say my $.02 and dump it. Y'all enjoy yourselves when I'm done, but here's my shot of Thorazine for you as a guy with a pretty thorough education in military science:
The whole argument is (a) pointless, and (b) marred by massive amounts of simplistic thinking on both sides. OK, so Lincoln didn't go to war to free the slaves, which is one of the biggest mistake the Union advocates make, thanks to public skrool.
The Southern partisans have their own counterpart to the 'Lincoln freed the slaves' error in their identification of everything in the North with Lincoln and his personal beliefs. Freeing the slaves may not be why Lincoln went to war, but is sure is shit was why an awful lot of the Northern electorate supported the war and were willing to go at it once SC flung down the gauntlet. The age of the divine right of kings was well past, Lincoln was the Chief Executive of a democracy with separation of powers, war was NOT going to occur on his will alone. He wasn't going to be going to war to preserve the Union or any other reason without a consensus for that in Congress, and after Sumter, that was overwhelmingly present, demanded actually. Lincoln was a moderate compared to a good chunk of Congress, most of whom were far more eager than he was go at it. Some to suppress insurrection and preserve the Union, some to seize the opportunity to end slavery, some to answer an attack, and certainly some for less noble reasons such as settling scores. Their individual motives are lost in the stew of the final collective move by Congress and the President deciding for war.
As far as 'invasion' goes, or in terms less appealing to Southern partisans 150 years later, 'suppressing an armed insurrection' (Which actually WAS contemplated in the Constitution in Article I, Section 8 - a document that nowhere mentions secession, I might add), those events were quite successfully put in train by the aggressively warlike stance of Secessionists in South Carolina. If you want to ensure there's a war, the damn-sure easiest way to do it is to start the shooting, which they did. No national government, certainly in the 19th Century, is going to put up with that shit. Basing your plans on a contrary belief truly is magical thinking.
It's fairly clear when you read about it that until that point the North was in a Mexican standoff with the South, and most immediately concerned about seizures of Federal property, but not willing to raise the stakes to overt armed action first. SC saved them a lot of dithering and negotiation. Nice going, sow the wind and reap the whirlwind, don't start something you can't finish, etc., etc., etc. all apply here.
Civil War buffs love the tactics, but the South failed at a strategic level, neutralizing any initial tactical advantage. Strategically, the South ultimately had a mobilization rate that was at the limit of what a country can sustain, up to around 20%; the North never did, theirs was more like 2-3%...and even with that, their field forces were at least double the South's, and lavishly outfitted by comparison to boot (Offset theoretically by the South's interior lines, which given their comparatively primitive rail net wasn't much of an advantage in reality). Talking about man-for-man, sure, the South had an advantage at first, but only an idiot or a desperate man takes the offensive on 1-to-1 odds, so it was never man-for-man. The entire object of maneuver is to concentrate forces so that at the point you wish to attack, you have locally-superior forces, and as many other advantages as you can orchestrate. And, that qualitative difference shifted as the war progressed as the South was ground down and the North raised their standards, both for common soldiers and generalship, and those tables had started turning the other way by mid-1863. About the best major engagement the South ever had in terms of loss ratios was Fredericksburg, a great tactical victory (Yet a defensive one), but the South could have had that level of trade-off in every battle they ever fought and STILL would have lost the war on a strategic level.
The South's strategic vision (To the extent they had one, they had a serious shortage of stategic thinkers in leadership positions) on which their hopes were pinned proved to be badly misjudged, since it depended on the importance of cotton (primarily) and its other products on the world market, and to the European superpowers of England and France in particular. At least the Egyptians benefitted from that gross error, though it cost the South pretty dearly. After the European powers failed to back their play, the South actually had no strategic plan or path to win, and its flawed strategic attempts to take the war to the North ended in failures ranging from disappointing to disastrous in 1862 and 1863.
The only way they were going to come out of it 'successfully' was to make taking the South such a bloodbath that the North would lose its will to prosecute the war, which they ultimately failed to do. The last gasp of that forlorn hope should have been crushed like a porn star's dreams when Lincoln was re-elected in 1864, but they kept fighting to no further purpose into the next year.
