The near extermination of the Native Americans has to be #1. Conflicts and killings were
to be expected but the systematic round-up and murder was a crime against humanity.
The troops that committed these crimes came right out of the killing fields of the Civil War.
Lincoln's assassination, Stevens his VP was illiterate until his early 20s, Grant, a good general, but very weak president, the drive of our expansion of territory with the transcontinental railroad, all these things made it happen.
But who cared? We were entering The Gilded Age. Our first of several post war economic
booms. ( Cadillac Escalade anyone?)
Unlike what we did to the natives, we own that 100%.
Slavery we inherited, its a crying shame it was not outlawed during the Continental Congress. It was said,"Slavery laid under the table as a coiled snake". Jefferson said,"Its
like holding a wolf by the ears, you don't like it but you dare not let go". They knew it was wrong! Slavery is so close a second the it could be a tie.
Atomic weapons being dropped on Japan?
Read, Hirohito and The Making of Modern Japan
The Last Mission
Eagle Against The Sun
The Rape of Nanking
The Last Mission, documents very well what the Japanese were willing to do to keep us off their home island.
The fire bombing of Tokyo was the major air strike. That should have been the wake up call.
In declared war all bets are off.