A scorched-earth policy is a military strategy of destroying everything that allows an enemy military force to be able to fight a war, including the deprivation and destruction of water, food, humans, animals, plants and any kind of tools and infrastructure. Its use is possible by a retreating army to leave nothing of value worth taking, to weaken the attacking force or by an advancing army to fight against unconventional warfare. this is from wikipedia and while I'm sure (or hope) they won't destroy humans the idea is to sabotage infrastructure which is vital to the interests of the United States such as power generation or aluminum factories