A scorched-earth policy is a military strategy that targets anything that might be useful to the enemy while advancing through or withdrawing from an area. Specifically, all of the assets that are used or can be used by the enemy are targeted, such as food sources, transportation, communications, industrial resources, and even the people in the area.
The practice can be carried out by the military in enemy territory, or in its own home territory. It may overlap with, but is not the same as, punitive destruction of the enemy's resources, which is done for purely strategic/political reasons rather than strategic/operational reasons.
Notable historic examples of scorched-earth tactics include the Russian army's strategy during the failed Swedish invasion of Russia, failed Napoleonic invasion of Russia, William Tecumseh Sherman's March to the Sea in the American Civil War, Lord Kitchener's advance against the Boers, the initial Soviet retreat commanded by Joseph Stalin during the German Army's invasion of the Soviet Union in the Second World War, as well as the subsequent Nazi German retreat on the Eastern Front.
The strategy of destroying the food and water supply of the civilian population in an area of conflict has been banned under Article 54 of of the 1977 Geneva Conventions. The relevant passage says:
It is prohibited to attack, destroy, remove, or render useless objects indispensable to the survival of the civilian population, such as foodstuffs, agricultural areas for the production of foodstuffs, crops, livestock, drinking water installations and supplies, and irrigation works, for the specific purpose of denying them for their sustenance value to the civilian population or to the adverse Party, whatever the motive, whether in order to starve out civilians, to cause them to move away, or for any other motive.