Their mission is described as safeguarding the country's frontiers against penetration or illegal crossing. Because they are a part of the regular armed forces, it is presumed that in time of war they would work in coordination with those forces. If the enemy were to penetrate into Bulgaria, the Border Troops would be expected to control the area immediately behind the ground forces. If Bulgarian armies were driving the enemy beyond the borders, they would probably remain at the old border or establish a new one if the leadership expected to retain any newly occupied territory. The most strictly defended borders are those shared with Greece, Turkey, and Yugoslavia, but the border with Romania is also defended. The Border Troops operate a number of patrol boats, both on the Danube River, where it forms the border with Romania, and along the Black Sea coast. The troops also control the movement of people into and within a border zone, which is a strip approximately eight miles wide in from the border. Smuggling, however, even large-scale smuggling, is the concern of the Ministry of Internal Affairs customs police and not of the Border Troops. Construction Troops A Bulgarian institution that is unique among the Eastern European communist countries is the organization known as the Construction Troops. Thousands of young men who are not called for service in the regular armed forces are drafted into the Construction Troops, from which the government derives productive labor at the same time that it instills military discipline and political indoctrination into a large segment of the young male population. Similar organizations have been maintained since the establishment of the original Labor Service in the early 1920s, which was a means of circumventing the World War I peace terms that prohibited large conscript military forces. Obligatory military service was restored during the 1930s and, as part of the change, the Labor Service was militarized. It was made a part of the army and remained so during World War II, when it became known as the Labor Army.