Drain the swamp
is a military euphemism for a particular type of anti-guerrilla warfare
. Since guerillas usually live off the land, either raid
ing villages or receiving aid from them, removing the villages forces the guerrillas to leave or be exposed
. The phrase probably derives from one of the tactics used to kill mosquito
s - by draining the swamps where they breed.
Similar military tactics are urbanization, pacification, and concentration camps. Historically, these have all been used as a means of "draining the swamp" by localizing the rebellious population, usually delineated by ethnicity or religion, within an area easier to control.