The last time the US had a Neptune opposition (Sibly Chart) was in the late 1850s, in the run-up to the Civil War. The natal square from Neptune to Mars goes some way to explaining why the country gets so polarised under this transit. Under the last opposition, the issue involved the redemption (Neptune) of the slaves and, with it, the soul of the country. This time Donald Trump is, for many, the Redeemer (he currently has Neptune squaring both his Sun and Moon), while for others he is the Scapegoat (another Neptunian theme) - and I am not saying he is not a good hook for everything we disapprove of in America, but a hook nonetheless. The US is also coming up to a Chiron Return, a healing theme as well as a scapegoat theme. When the US Civil War began, the Prog US Asc was at 16 Pisces. Currently, the Prog US Sun is at 16 Pisces. Pisces is ruled by Neptune.
One way of describing a Neptune transit is loss and renewal of self, and the US has done this on both occasions of the opposition through polarising initially, though we have yet to see a current renewal. Don't underestimate the strength of feeling, the anger, in the US at present over the 'stealing' of the election, deluded as it may be (tr Neptune square natal Mars). But we are not talking reason, we are talking religious experience/belief (natal 9th House Neptune opp tr Neptune, Prog Jupiter opp Prog Pluto), which is not amenable to reason.
The Neptune opposition has a while to run and so, I am sure, will the consequences of the 'stolen' election run on. It provides part of the context for the US Pluto Return in 2022, which will complete a cycle of American economic power, turbocharged in the late 1930s under the Pluto opposition as the US emerged from the Great Depression and came to dominate the world via WWII. This time the US will be gradually ceding economic dominance to China, raising questions of its economic power-based identity (2nd House natal Pluto). The deeper anxieties expressed by the current Neptunian polarisation perhaps prefigure the power/identity issues that will be raised by the Pluto Return. Who are you when you only know how to be the winner: the theme for Trump right now, and for the US in the near future.