Can the polarization between Democrats and Republicans in the U.S. be reversed?

[Angrboda]

I strongly suspect that the reconciliation in the Civil War is not the only such example in world history, but in any case, you have the arrows reversed.  The people who made concessions were those who had triumphed militarily and thus had no need to make concessions, not those who might have been cowed by their prior defeat.  So you have the history here upside down.

No, the people who made the concession were the people who wanted to leave the union and keep slaves.  They did not make the concession.  The concession was taken from them.   The fact that they then waged a century long political war to regain parts of what they conceded and appears to even recently be making progress that cheerier view of history had not allowed to be possible does not reverse the overall balance.

No, you're wrong. The rapprochement was a result of a concession from the northern Republicans and a failure to secure legislation contrary to southern interests. Neither were a result of southern Democrats giving up anything. The intolerance between southern Democrats and northern Republicans was not ended by the Civil War, although the southern Democrats did give up slavery as a result of their loss of the war. The Civil War continued, just through, in Clausewitz' phrase, other means.

Gradually, though, as the Civil War generation passed from the scene, Democrats and Republicans learned to live with one another. They heeded the words of former House Speaker James Blaine, who in 1880 advised fellow Republicans to “fold up the bloody shirt” and shift the debate to economic issues.

It was not just time, however, that healed partisan wounds. Mutual toleration was established only after the issue of racial equality was removed from the political agenda. Two events were critical in this regard. The first was the infamous Compromise of 1877, which ended the 1876 presidential election dispute and elevated Republican Rutherford B. Hayes to the presidency in exchange for a promise to remove federal troops from the South. The pact effectively ended Reconstruction, which, by stripping away hard-fought federal protections for African Americans, allowed southern Democrats to undo basic democratic rights and consolidate single-party rule. The second event was the failure of Henry Cabot Lodge’s 1890 Federal Elections Bill, which would have allowed federal oversight of congressional elections to ensure the realization of black suffrage. The bill’s failure ended federal efforts to protect African American voting rights in the South, thereby ensuring their demise.

— "How Democracies Die," Steven Levitsky and Daniel Ziblatt.