I think you underestimate the internal divisions that showed during the civil war. They're still there.
It's not like the Compromise of 1850 erased all previous internal divisions.
Jackson's "Our Federal Union! It Must Be Preserved!" vs Calhoun's "Our Federal Union — next to our liberties the most dear!" was 1830.
The russians aren't creating divisions, they are strengthening existing ones. Any existing division can be blown up to be all en-compassing. Very easy to tap into into everyone's deep wired in-group / out-group behavior. With enough effort and fuel you can have people willing to secede from the USA because of very strongly held feelings about pineapple on pizza.