I think the best argument one could make in favor of displaying the Confederate battle flag would go something like this:
“By the time of Fort Sumter, the C.S.A. was an independent nation that had formed a government and controlled large amounts of territory. That nation was invaded by a foreign army — just as, in 1776, the American colonies-turned-states (in every one of which, north and south, slavery was legal at the time of independence) were invaded by Britain. Southerners responded valorously in 1861, just as their ancestors had in the Revolution.
“If the United States had lost the Revolutionary War, the bravery of its fallen soldiers would be no less worth remembering today, the way we remember those we lost in Vietnam and in this century’s wars. And just as it is appropriate to fly the Bennington flag over a Revolutionary War cemetery, so too it is appropriate to fly a Confederate flag over a memorial to Confederate dead.”
But, like the Lost Cause itself, this argument is not quite strong enough to hold up. Regardless of how its common soldiers may have felt, the Confederacy’s raison d’être was slavery, not the euphemistic “states’ rights”; no state, not even South Carolina, would have seceded over a tariff. While the Confederate flag holds different meanings for different people, nowadays the connection with slavery is uppermost in many, perhaps most, people’s minds, and for that reason, it has become inappropriate for display in public places.