This list suffers from a powerful recency bias, but of the dates listed (and probably of those not listed) I voted for 1862, which was arguably the last time when the U.S. could plausibly have ceased to exist.
Even if the Confederacy had won it is not like the rest of the United States would have broken up? The country would obviously be a lot different, but it is not like the US would cease to exist. If anything I'd say the last time the US could have ceased to exist was when it was fighting the war of independence (even the 1812 war was in self-defence from Canada's perspective and Britain had 0 interest in reconquering the US?)
The US would still exist to this day, with still most of their current territory (minus the confederacy obviously) and would have still been the number 1 superpower for much of the 20th century, though the Cold War would have been closer and I suppose China would have already beaten the US in terms of power at the turn of the century but the US would still be a comfortable number 2.
That all assumes history still goes through a similar path as we know beyond the Civil War of course which is a big stretch but still.
Poll
