SOUTH SUDAN: Swooping low over the banks of a Nile River tributary, an aid flight run by retired American military officers released a stream of food-stuffed sacks over a town emptied by fighting in South Sudan, a country wracked by conflict. Last week’s air drop was the latest in a controversial development: private contracting firms led by former US intelligence officers and military veterans delivering aid to some of the world’s deadliest conflict zones, in operations organized with governments that are combatants in the conflicts.