The Territorial Defense (TO) were a separate part of the armed forces of the former Socialist Federal Republic of Yugoslavia. The forces acted as a Home Guard which roughly corresponded to a military reserve force or an official governmental paramilitary. Each of the Yugoslav constituent republics had its own Territorial Defense military formations, while the regular army for the whole Federation was the Yugoslav People's Army (JNA), which also maintained its own reserve forces.
Yugoslavia was a socialist state but not an Eastern Bloc country. In 1948, following the Tito–Stalin split, Yugoslavia broke ties with the Soviet Union and its allies. During the Cold War, it was one of the leading members of the Non-Aligned Movement. After the Soviet Invasion of Czechoslovakia in 1968, the concerns about an eventual Soviet attack started to rise within the Yugoslav leadership. The invasion of Czechoslovakia showed that the standing conventional forces of a small country could not repulse a surprise attack by a qualitatively and quantitatively superior aggressor. Being strategically positioned between the two major blocs, the NATO and the Warsaw pact, Yugoslavia had to prepare its own military doctrine for an eventual Third World War scenario.
With the passing of the National Defense Law of 1969, Yugoslavia adopted a total war military doctrine named Total National Defense or Total People's Defense (ONO). It was inspired by the Yugoslav People's Liberation War against the fascist occupiers and their collaborators in the Second World War, and was designed to allow Yugoslavia to maintain or eventually reestablish its independent and non-aligned status should an invasion occur. According to it, any citizen who resists an aggressor is a member of the armed forces, thus the whole population could be turned into a monolithic resistance army.
Starting from the elementary school education, over high schools, universities, organizations and companies, the authorities prepared the entire population to contest an eventual occupation of the country and finally to liberate it. For this purpose, the Territorial Defense (TO) would be formed to mobilize the population in case of an aggression. The combat readiness of the TO meant that the steps of organization and training could be bypassed after the start of hostilities. The TO would supplement the regular JNA, giving it greater defensive depth and an armed local population ready to support combat actions. Large numbers of armed civilians would increase the cost of an invasion to a potential aggressor.
The most likely scenario in the doctrine of ONO was a general war between the NATO and the Warsaw Pact. In such a situation, Yugoslavia would remain non-aligned, and it would not accept foreign troops of either alliance on its territory. The doctrine did recognize the likelihood that one side or the other might try to seize Yugoslav territory as a forward staging area, to ensure lines of communication, or simply to deny the territory to enemy forces. Such action would be considered aggression and would be resisted. Regardless of ideology, the occupiers would be considered Yugoslavia's enemy.
The Territorial Defense or TO were formed in 1969 as an integral part of the Yugoslav Total National Defense doctrine. The TO forces consisted of able-bodied civilian males and females. Between 1 and 3 million Yugoslavs between the ages of 15 and 65 would fight under TO command as irregular or guerrilla forces in wartime. In peacetime, however, about 860,000 TO troops were involved in military training and other activities.
The TO concept focused on small, lightly armed infantry units fighting defensive actions on a familiar local terrain. A typical unit was a company-sized detachment. More than 2,000 communes, factories, and other enterprises organized such units, which would fight in their home areas, maintaining local defense production essential to the overall war effort. The TO also included some larger, more heavily equipped units with wider operational responsibilities. The TO battalions and regiments operated in regional areas with artillery and antiaircraft guns and some armoured vehicles. Using their mobility and tactical initiative, these units would attempt to alleviate the pressure of enemy armored columns and air strikes on smaller TO units. In the coastal regions, TO units had naval missions. They operated some gunboats in support of navy operations. They were organized to defend strategic coastal areas and naval facilities against enemy amphibious landings and raids. They also trained some divers for use in sabotage and other special operations.
The TO was helped by the fact that most of its citizen-soldiers were one-time JNA conscripts who had completed their term of compulsory military service. But TO recruitment was somewhat limited by the JNA desire to include as many recently released conscripts as possible in its own military reserve. Other sources of TO manpower lacked prior military service and required extensive basic training.
The TO was highly decentralized and independent. TO units were organized and funded by the governments in each of the Yugoslav constituent republics: SR Bosnia and Herzegovina, SR Croatia, SR Macedonia, SR Montenegro, SR Serbia and SR Slovenia, as well as in each of SR Serbia's subunits SAP Vojvodina and SAP Kosovo.
The possibility that each Yugoslav federal unit could have its own armed formations led to concerns that someday these separate "armies" may oppose the federal Yugoslav JNA in an act of an eventual secession. Such concerns became reality during the breakup of Yugoslavia and the Yugoslav Wars when the TO forces in many of the constituent republics switched their allegiance and turned into separatist paramilitaries. Those former TO forces, along with Yugoslav army deserters and volunteers contributed to the founding of the respective armies of the independent states and other political entities that emerged after the disintegration of Yugoslavia. This includes the armies of Bosnia and Herzegovina, Croatia, Macedonia, Montenegro, Serbia and Slovenia.