Many U.S. servicemen fought under the Confederate battle flag during World War II. The fighting ability of the Confederates is well recorded in history... even the Germans feared it!!! Whenever they saw that flag, they knew they were in for a fight.
I think mostly tank crews painted it on the side of their tanks.
I remember seeing a movie...could've been Midway , where one of the aircraft carriers had a Confederate battle flag flying from the topsail. (CSN battle ensign)
I believe this flag was in common use during WWII on all fronts. It is an American flag, and represents American ideals, values and courage. Infantry adopted a flag which was unmistakable in battle, because the First National Flag looked too much like the Union forces' flag. And the Second National Flag resembled a white flag of surrender. So, they adopted the naval jack of the CSN, which was at first was only flown from warships, i.e. Alabama, Florida (brigantines), and Virginia, Tennessee (ironclads.)
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The reasons for the Civil War (War Between the States) are very complex. Slavery, while a "cause de guerre" in the final conflict, was one of many issues leading up to the war. You would just about have to be a PhD in History to delve into this very complex study, and even then, there would be disagreements.
I believe the states' rights issue had much to do with taxation without representation, and was probably viewed by the Southern states as a hostile business tactic employed by the seat of the Union in Virginia (Washington), to squeeze tax money from the profits of slave labor, at the same time, counting Negro slaves as 3/5 rather than full "votes" in the U.S. Constitution. Thus short-changing the South on its representational strength by population. Not exactly fair, but accepted by Yankees.
The South still has not recovered from the Civil War, due to Reconstruction...the price paid for losing the most bitter war in American history.
Les