Good example.
A recent study estimated the population of England was reduced during the wars by about 10%, Scotland by about 15% and Ireland by about 20%. Obviously the vast majority of these were civilians, as the armies just weren't that large.
An equivalent loss in the USA of the 1860s (comparing the 10% figure) would have been about 3M fewer Americans in 1870 than 1860.
In actual fact, of course, the US population increased during the 1860s from 32M to 38M, about 22%. With the 38M figure widely considered at the time and since to be inaccurate on the low side.
In the English Civil Wars many if not most civilians died not from massacre or intentional killing, but rather from malnutrition, disease, etc.
But of course these killers were also potentially present during our war.
Given that most deaths to due starvation and malnutrition in any area are normally caused indirectly by damage to transport infrastructure. The quirks that most of the war was fought in the south and the south was closer to semi-subsitance than the north and that most of the damage to the transport systems was able to be fixed quickly, kept deaths from malnutrition and starvation down.
Disease is another cause that gets interesting, and US quirks may have had an effect on this. Communicable diseases traditionally decimated the farm boy conscripts that were the backbone of the armies before the industrial revolution. Much of this could be traced to many of them not gaining immunity to "childhood" diseases before entering the army. Any epidemic that started in the camp could be expected to affect nearby civilian populations. Armies on the move could and did spread epidemic around, A saying from the first world war was that it was more dangerous to shake a Russians hand than to have him shoot at you (typhus possibly)
Well, the point here is military THEORY. Sherman and Sheridan were the first, in modern times to carry out a military theory of deliberately targetting civil populations, as opposed to the earlier idea of the objective of war being to destroy the enemy’s army and capture his strongholds.