In June 1945 the 1st, 3rd and 8th Infantry Divisions were assigned internal security duties. The 4th Infantry Division was reorganised for the purpose of creating the Internal Security Corps (KBW). The rule was that military units were used primarily against the Ukrainian Insurgent Army (UPA), while the Internal Security Corps was used to fight the armed underground independence. Often however army units fought the underground resistance, and vice versa. The culmination of the UPA suppression operation was the so-called 'Wisła Action' (Operation Vistula) which took place in 1947. At the same time demobilisation took place, moving the armed forces to a peacetime footing. On 10 August 1945 a "decree of the partial demobilisation" of the armed forces was issued. The next demobilisation phases took place in February and December 1946. One of the most important tasks facing the army after the war was mine clearance. Between 1944 and 1956 the demining operation involved 44 engineering units or about 19,000 sappers. They cleared mines and other munitions in a clearance area of more than 250,000 square kilometers (80% of the country). 14.75 million munitions of various types and 59 million bullets, bombs and other ammunition were found and removed. The mining operations cost the lives of 646 sappers. In 1949 the military districts were reduced to four. They were the Pomeranian Military District, HQ in Bydgoszcz, the Silesian Military District, HQ in Wroclaw, the Warsaw Military District, HQ in Warsaw, and the Kraków Military District with its headquarters in Kraków. In November 1953, the Kraków Military District was dissolved and until 1992, Poland was divided into three districts. Following victory and the movement of Polish borders these troops and other Polish soldiers thought loyal to their Soviet overlords were built up into a force which was to form part of the Warsaw Pact. Polish Army troops would have formed part of the second strategic echelon deployed for an attack on NATO's Allied Forces Central Europe. A Polish Front headquarters was formed in 1958, along with three armies formed from 1955, the First Polish Army, the Second Army, and the Fourth Army, mobilisation-only headquarters that were to be formed within the three districts. The Polish Front headquarters was deactivated in 1990, and the three-army mobilisation scheme was likewise abandoned. Polish land forces during the communist era included troops dedicated to internal security – the Territorial Defence Forces – and control of