Wholesale pillage. This was done by both North and South, though the North remains more famous for it. Especially in the western theater, as the war dragged on, Union troops became more hardened toward the population of the South, while at the same time their generals (Grant and Sherman, but A.J. Smith and Phil Sheridan bear mention) realized that an army can move considerably faster and farther when it is not dependent on a lengthy supply train, but acquires its provisions from the farms and towns along its route of march. This eventually led to Sherman's devastating campaigns through Georgia and South Carolina, carried out for the intent purpose of rendering the South incapable of sustaining the war.

But, and this is very important to note, the South was far from blameless in this regard. Whenever a Southern army went north, goods and crops were confiscated in exchange for Confederate paper currency, with the theoretical promise that it could be redeemed for gold when the war was won. Stunningly, the Confederacy was forced to carry on in a similar manner in its own territory in order to feed the armies. Northern Virginia in particular was stripped bare to feed Lee's army, and its devastation caused logistical problems for the Confederacy later in the war. Patriotism is fine and dandy, but farmers can't live on your near-worthless, hyper-inflationary currency when you strip their fields and pastures bare.

Correct me if I'm wrong but the conditions at POW camps on both sides were absolutely abysmal. Certainly not within the grounds of the Geneva Convention or any other rule of law documents the US was a part of in the 20th and 21st century.