Why is America one of, if not the, only nations that experienced a non-religious Civil War in the past 200 years in which the losing faction is still obsessed with its iconography?

The simple answer is that it didn’t: the US did not experience a non-religious Civil War. The more complicated answer is that this period arguably sees the maturation of the US civil religion that conflates a unitarian god with politics, and, as can be argued, the period also sees the splintering of a Southern civil religion. More broadly, Mainline Protestant churches and Evangelical Christianity are intensely involved in both the lead up and the progression of the war, and they are fundamentally altered by it. Indeed, as I will return to later, many US churches schism over slavery, forming a Northern and a Southern denomination. This period also sees a huge intensification of the religious life: from this period forward, the US will grow in formal church affiliation for the next century, cresting in the 1950s and 60s before beginning to decline.

The language and mythology surrounding the Civil War is positively suffused with religious imagery and iconography on both sides. In the North, Lincoln is an obvious starting point. Lincoln is a deeply religious man, despite much of what has been said about him claiming the contrary. At most, Lincoln is a vaguely humanist religionist earlier and a more traditional unitarian (though hardly Christological) Christian later. Lincoln’s Second Inaugural cuts to the very deeply religiously inflected nature of the war:

If we shall suppose that American slavery is one of those offenses which, in the providence of God, must needs come, but which, having continued through His appointed time, He now wills to remove, and that He gives to both North and South this terrible war as the woe due to those by whom the offense came, shall we discern therein any departure from those divine attributes which the believers in a living God always ascribe to Him? Fondly do we hope, fervently do we pray, that this mighty scourge of war may speedily pass away. Yet, if God wills that it continue…, so still it must be said "the judgments of the Lord are true and righteous altogether.

Lincoln popularizes a sort of baptism by blood narrative between this and the Gettysburg Address: the sins of the country have been great, and the ultimate sacrifice of blood is what will cleanse the country of its sins. Of course, this is all the more intensified by his own assassination, which places him firmly as the Christ-figure in that narrative. And this is absolutely something people bought into: since the very beginning, (future) Americans have been taken by Biblical typology. Typology looks at the Old Testament of the Christian Bible and sees “types” that it also identifies in the NT. In the colonies, this extended into their own lives: they were the Israelites and George III was Herod, for example. Similarly, the Second Great Awakening has just birthed Dispensationalism, which saw history as divided into periods (dispensations) governed by specific ideas. Premillennial dispensationalists of the period think they are in the end of one dispensation, which will culminate in horrors and tribulations before giving way to the 1,000-year reign of Christ (bloodiest war in US history, anyone?).

For many typologists, the US was basically “Israel 2: New and Improved Formula” (not unlike present-day Evangelicals’ view of modern Israel). It was going to do what the first one could not and actually effect the coming of Christ. You actually have very bizarre arguments that Ancient Israel was the first democracy, etc. So basically, the art of liberty had been lost, and the US would restore it. The US was variously the model for the rest of the world or the one who would actually make it happen (not just show people how to do it), the latter maturing after the war as the US became an imperial power. Thus for many Northern Protestants (and some Southern ones), secession and civil war threatened the bastion of democracy and the bringer of the Kingdom of God. The war was, potentially, the war against the antichrist. If not literally, than it at least represented that fundamental struggle between the forces for God and good and for the Devil and evil.

Another heavily important aspect to this is the role of Evangelical Christianity. Harriet Beecher Stowe, of a very famous Evangelical family and herself very deeply Evangelical, writes basically the Oprah’s Book Club choice par excellence of her time in Uncle Tom’s Cabin. It disgusts many Northerners into action (perhaps the cell phone video of police brutality of her time as well), and it represents her and other Evangelicals’ fervent belief in the value of each individual, their full spiritual equality, and their full potential for being saved and entering the Kingdom of Heaven (not to mention proselytizing on Earth). Evangelicals are (perhaps paradoxically from our vantage) heavily involved in progressive pursuits and are some of the most ardent abolitionists. Evangelicals of the time are tireless advocates for abolition (though we should remember they often held views that were still racist by today’s standards). They tended to practice (and still do) a less-organized non-denominational form of Christianity, however, so we don’t see many of the splits, nor much of the mobilizing power, of the large mainline denominations. But this is a major turning point for the consolidation of Evangelical organizations (like voluntary and missionary societies), and the maturation of a more coalesced movement for Evangelicalism.

For the Mainline, the antebellum period is rife with distress. Mirroring what many have feared may be on the horizon today for the UMC over homosexuality, the Methodist Church of the time split in the early 1800s because of slavery, splintering one of the largest denominations in the country. Almost one hundred years passed before the resulting two branches reunited. The Baptists and the Presbyterians similarly broke in half along north-south lines. So you really do have, in essence, Northern churches battling Southern churches. The only difference between the two denominations is the American Protestant affinity for splintering denominations by adding words like “united” or “in Christ” and, far more importantly, support for slavery.

That is all to say that in any way any other war is religious, the US Civil War was too: no war is ever truly fought because of religion sui generis. There is always a complex interplay of political and religious motives. The way religion is ushered in to make sense of the Civil War is, I stridently argue, a definitional facet of American war. Americans understand war through religion and religious themes: before, during, and/or after the fact. And in the case of the Civil War, it was all three: religion made meaning for slavery (either as abomination or holy charge), it made meaning of the war as a holy one fighting for the soul of America, and it was ushered in to heal divisions after the fact.

But I’ve spent all this time refuting the basis of your question, not answering it in a way it can be answered. The best answer for that is the Myth of the Lost Cause. Following the Civil War, you have a period of relative dominion of the North over the South. At first, Johnson (who was the only Southern US Senator not to secede, and was selected as Lincoln’s VP because he was fiercely pro-Union) obfuscates any efforts to punish the South or help Black Americans. But because he stymies things so badly, Congress swings heavily Republican (obviously, fully different party from the current one) and begins overriding his vetoes and enacting pretty radically Republican legislation. By the 1870s, however, people start to tire of division and want to move past the war. Enter the Myth of the Lost Cause.

Near the end of the century, a reclamation of the South’s legacy goes into full swing. Their cause was a noble one, their loss a valiant sacrifice. The Southern culture becomes identified heavily with a genteel chivalrousness: men protected their wives and daughters, raised good, strong sons, and looked after their slaves. Vicious beatings and murders are portrayed as few and far between, and slavery is rebranded as actually probably not that bad.

The South starts to develop its own sub-civil religion that paradoxically paints armed insurrection against the federal government as true patriotism. This makes sense in their schematization because they argue they were fighting for the true basis of US Government federalism that ensures state’s (and thus the people’s) sovereignty. Lincoln goes from being vilified to being the sacrificial lamb as he was in the North, for one, and the state’s rights myth explodes in popularity.

Southerners, perhaps more so than other Americans, retain deep sectional identities. Unfortunately, the most unique markers of Southern identity are those associated with the Civil War for obvious reasons. Returning to typology and the broader American mythos, identifying founding fathers is a mythical and identity-affirming pursuit: just as the broader culture sees the Founding Fathers as emblematic of their very identity, so do Southerners see the Confederate leaders in that way. OT patriarchs to Apostles to Founding Fathers to Confederates. Indeed, the way the Confederate flag is worshipped is no different from the way the US flag is worshipped. In both cases, a kind of idolatry imbues this physical object with whatever mythical and abstract qualities the beholder associates with the country it represents. To damage or denigrate the flag is to harm the ideas it represents directly.