The Ethiopian army grew considerably under the Derg (1974–1987), and the People's Democratic Republic of Ethiopia under Mengistu (1987–1991), especially during the latter regime. The Library of Congress estimated forces under arms in 1974 at 41,000. By July 1975 the International Institute for Strategic Studies was listing a mechanised division in addition to three infantry divisions. Ayele writes that in November 1975, the "Nabalbal" ("Flame") force was created, subdivided into battalion-sized units of 400. Each battalion-sized unit was known as a "hayl" (force), and 20 were created within sixteen months. The "Nabalbal" units entered combat in 1977. When Ethiopian intelligence sources discovered Somali planning to seize the Ogaden, militia brigades were also created; first 30, then a total of 61 brigades totalling 143,350 by 1977–78. It appears that there were five regular line divisions active by the time of the 1977 Ogaden War, and the Library of Congress estimated the force size at the time as 53,500. With significant Soviet assistance, after that point the army's size grew rapidly; in 1979 it was estimated at 65,000. The 18th and 19th Mountain Infantry Divisions were then established in 1979-80 originally to seize Nakfa, in the Sahel Mountains, one of the remaining strongholds of the Eritrean insurgents. By the beginning of 1981 recruitment for the 21st and 22nd Mountain Infantry Divisions was underway; soon afterwards, preparations for the large Operation Red Star were stepping up.