The Mobile Army Surgical Hospital (M.A.S.H.) Sector is a neutral zone in No Man's Land created by Leslie Thompkins to provide medical assistance and shelter to those left within the city limits when the American army demolished the city's bridges.
Located around the Crime Alley district of Gotham where Thompkin's medical practise was originally located, the sector's neutrality was widely respected by the various factions in the city and was seldom challenged. Eventually the sector came under the unofficial protection of Batman upon his return to the city.
Following the reestablishment of Gotham City's power base, the sector was reinforced by the United States Army Medical Forces and then later dissolved when Gotham's medical facilities were reinstated.