Anti-terrorist Operation Zone

Anti-Terrorist Operation Zone (Ukrainian: Зона проведення антитерористичної операції, romanized: Zona provedennya antyterorystychnoyi operatsiyi), or ATO zone (Ukrainian: Зона АТО, romanized: Zona ATO), was a term used by the media, public, the government of Ukraine, and the OSCE and other foreign institutions to identify Ukrainian territory of the Donetsk and Luhansk regions (oblasts) under the control of Russian military forces and pro-Russian separatists. As the 2014 pro-Russian unrest in Ukraine escalated with the arrival of armed militias, the Ukrainian government declared the start of an "Anti-Terrorist Operation" (ATO), marking the beginning of the War in Donbas.

Source: Wikipedia — Anti-terrorist Operation Zone (CC BY-SA 4.0)