It seems to me that until the Civil War, Evangelical Christianity was definitely a northern thing and the South was more relaxed. Was it just the disaster of the war that changed southern religious outlook?
I suppose that depends on how you're defining "Evangelical Christianity". The "northern thing" was the preponderance of Reformed Protestant churches i.e. Presbyterian, Lutheran, Methodist, and Episcopalian that went along with the cultural heritage of the original colonies, and the country expanded westward and southward IMO faster than those particular churches did. They didn't call it the "lawless West" for nothing.
That's actually a very astute question. I'll share what little I know from reading history.
The original American "Bible Belt" was not the South (which was Anglican and didn't take its religion very seriously) but New England (now the most anti-religion section of the country). The First "Great Awakening" occurred in New England. It wasn't until the Second Great Awakening (early nineteenth century) that "evangelical" theology really hit the South. And btw, the early American Baptists adopted something called the "New Hampshire Confession of Faith" which today the Southern Baptist Convention conforms to much closer than the American (formerly Northern) Baptist Convention--even though the SBC doesn't officially recognize the New Hampshire Confession, since they "have no creeds."
Early evangelical political activity was very much aimed at "reform." This included opposition to slavery, to alcohol, to Freemasonry, and to Catholicism. All this was at first a crusade of doctrinally orthodox people--but it didn't stay that way. As New England Calvinists burnt out and became Unitarians they kept their zeal for moral reform but jettisoned the orthodox theology on which it had originally been based.
Anti-Masonry is the archtypical example of this. What had started out as the original "red scare" (because Northern Federalist worries of the influence of French Jacobinism) eventually morphed into a form of radical anti-aristocracy and anti-clericalism. Since Masonry served the role of both an "aristocracy" and a "state church" in the United States (public ceremonies were often Masonic in a country that had no state church), it became the object of radical egalitarian and anti-clericalist attack (anti-Masonry is today regarded as a position of the Catholic Far Right but radical American anti-Masons were anti-Catholic as well). The grandchildren of the conservative Federalists and grandchildren of the Puritans had become secularized utopian "do-gooders" whose moral sensibilities no longer depended on G-d or religious dogma, but often sat in judgment on them.
I personally call this tendency of a position to migrate from one end of the spectrum to the other "ideological drift." Another example is the anti-alcohol, temperance, or prohibitionist position. In the nineteenth century this was a "left wing" modernizing reform movement that shared members with the abolitionists, the advocates of women's suffrage, the rights of labor, and world peace. Many notable nineteenth century reformers (and even radicals) started out in the temperance movement. Today the vast majority of temperance organizations are located in the states that made up the Confederacy and have the word "chrstian" in their names. Perhaps a Southern form of "Montezuma's revenge?" I could go into greater detail about the ideological connection of temperance to "left wing" reform, but won't do it here.
Another example of "ideological drift" from our time is the migration of support for Israel from the Left to the Right. Actually, this is a bit of a mis-statement for the very reason that devout Fundamentalist Protestants have always been Zionists, even all the way back to Increase Mather. I as a Fundamentalist chrstian Zionist was not even aware that liberals supported Israel at all, assuming that it simply fit in with the rest of the conservative Fundamentalist worldview and that universal Communist hostility to Zionism reflected the universal ideological pattern. The truth was so shocking to me that I've still never fully recovered from learning it, and probably never shall.
Even truly radical reformists, however, retained the concept of moralism. G-d might not exist (chas vechalilah!), but slavery was still "immoral." And in fact even the numerous "free love" advocates of the time objected to marriage, not because they regarded morality as old-fashioned and oppressive, but because they regarded it as a form of "legalized prostitution." And so far as I know, none of the proto-feminists of that era regarded abortion with anything other than horror.
Other than on alcohol, Israel, and (perhaps) Masonry, all the old reform causes have been radically secularized--in fact, now morality is seen as "oppressive."
Despite the assertions of neo-Confederate apologists, for the most part the North and the South shared the same basic religious beliefs (one did not have to be an atheist to oppose slavery, as the case of Orestes Brownson aptly demonstrates). However, this reforming impulse was less present in the South, perhaps (ironically) due to the Anglican influence (the Old South was also friendlier to Catholics and Jews than the evangelical North was at that time). This influence kept the South from becoming the Bible Belt until after the Civil War (in fact, some apologists for slavery insisted, like today's theological liberals, that Adam was not the father of all mankind; ie, only whites were descended from Adam). After the War and Reconstruction somehow the Southern Baptist Convention (organized 1845) became the "ex post fact" church of the Confederacy. I doubt very many decadent bourbon-swilling slave-holding aristocrats had been what we today call "Fundamentalist Baptists."
That'll have to do for now. I'm sorry, but I've been at the computer all day and simply need to see to a few other things. I hope you will find this useful, though.