The civil war changed that as the prevailing attitude. And that is the lens many mistakenly use when forming an opinion on the civil war. Even if slavery were the root of the war, the underlying disagreement was not about slavery (that difference had been brewing for decades by 1861). Rather, the disagreement dealt with a state's right to 'opt out' of the federal compact.
I suggest that during the formative years of Lee's youth (1820-1830), contemporary scholars would have taken for granted a state's right to leave...and the notion that a state couldn't was 'new' and downright revolutionary in 1861. Heck, most northerners didn't agree with Lincoln at that time, when it came to going to war over it.
Lee is not without fault, but far from a villain. Some people forget that an abolitionist hell bent on starting a war, by the name of Brown, was defeated at Harper's Ferry...by a then federal officer named Robert E Lee. Both sides were itching for a fight, and Lee did his duty the best he could...with his underlying allegiance to his state (as was customary in his youth) and not to the union.
ps: I'm the son of Yankees, who grew up in the south, and now lives in the state where the seeds of the civil war were sown - Kansas. I have relatives who fought for the union army (and one who fought on both sides). I am also a graduate of West Point (I mention because that seems to have become a theme of this thread). I have no blind loyalty to the south or the confederacy...but I feel I have a deep understanding of the southern psyche, and bristle at northerners who reflexively paint southerners as racists.
The secession of the Southern states meant that untaxed goods would be flowing directly from Europe to Southern ports. Since over 70% of Treasury revenue came from tariffs, and that as much as 75% of the tariff revenue was going to cease to exist, Lincoln did what he said he was going to do in his inaugural address.....force the South to pay.
Secession, slavery, etc. were just empty issues.
It was a tariff-trade war.