It is very difficult for me to decide which the most successful military campaign was. Unfortunately, there have been far too many successful military campaigns in that throughout history over a billion people have died in wars or in the aftermath of wars. Admittedly, some military actions were and are necessary to stop evil in the world such as was seen with Hitler and is currently being seen with ISIS and other terrorist groups.
However, if I have to choose, I would choose Sherman’s March to the Sea during the USA’s Civil War. Sherman’s march did not result in a lot of casualties but it killed the South’s will to continue fighting.