It was really only after Grant's victory at Vicksburg in 1863 that it became apparent to informed observers that the Union would prevail. Even so a substantial fraction of the North was in favor of a negotiated peace. George McClellan ran against Lincoln in 1864 on a peace platform. Bloody slaughters like the Union defeat at Cold Harbor in June 1864 made his platform very strong. McClellan lost but by most accounts Lincoln's campaign was saved by late victories in Georgia.
If you want to send an army into another country without local support you need a massive manpower and material advantage. After you account for that the two sides look more equal.
If Gettysburg had been a southern victory things could have turned out very differently.
Or you simply need to be willing to sack and burn the opponent until they starve.
In addition, the slaves, who outnumbered free southerners by quite a bit, were not necessarily unwilling to cooperate with the Union soldiers. That's very different from trying to pacify an area that will fight to last man.
I don't know that this is true. I think that the Lost Cause movement has perpetuated this based on pure numbers while ignoring all the other factors that could have impacted the outcome of the conflict. The North did have an overpowering industrial advantage, but that doesn't mean they had the desire to commit to a conflict. In fact, they spent the first 3 or 4 years of the war not committing fully.
If the Revolutionary War had failed, I think we would say the same thing about that, but a war is a complicated thing and I would never say an outcome is "clear to everyone". Clearly France saw some advantage in undermining an enemy, despite Britain's advantage over the colonies.