you were doing ok till you editorialized your own logic at the last paragraph
aside from seething resentments and strong cultural friction between north and south and economic issues aside from slavery, your first nuggets are fair enough but then you went subjective.
the expansion of slavery and what Lincoln’s election with a minority of the vote foreshadowed was the catalyst issue for both sides
preserving the union only became the focus for the north after the south started seceding after the election of Mr Lincoln...who was not actually a real abolitionist like Sumner or Stevens since as you correctly claimed did not intend to abolish slavery where it already existed.
after the South seceded, the North fought to keep them in the Union and the South fought to secede or more practically to fend off an invasion after they tried to secede
had the North done nothing, there is no hint the South would have invaded the north...and for what?....
You are right. The America west was the prize for both north and south. Reading the articles of secession makes that pretty clear, in my opinion. Had the south successfully seceded, things out west would have gotten much more interesting. Our wars out west which followed the civil war would have taken on a whole new dimension.
Would have made for some tremendously interesting history.
Lincoln offered them slavery if they gave up on western expansion. They refused explicitly because they intended to expand west. Its my opinion that civil war was inevitable. If not at Sumter, then out west. We would probably be four countries now. That might not be so bad, actually. But current history would be unrecognizable world wide for better or for worse. Some talented writers could probably have a hayday building a parallel history. It would be fun to take a crack at it, really.
...is everyone forgetting Fort Sumter?
Would the invasion have commenced with the support it had had the South not attacked Sumter?