The Israeli-Lebanese conflict commenced in the late 1970s, ending the relative stability of the demarcation line along that front. It provided the latest major change to UNTSO's structure. After the outbreak of the Civil War in Lebanon and the Israeli invasion of Southern Lebanon (March 1978), the United Nations established the United Nations Interim Force in Lebanon (UNIFIL). UNTSO's observers were thus reorganized into Observer Group Lebanon (OGL) to assist UNIFIL. September 1982 saw a low point for UNTSO with the deaths of four of its unarmed officer observers—one Finn, one Irishman, and two Americans—in a landmine explosion just outside Beirut. As the Israelis advanced north, the position of Observer Group Lebanon was adjusted. In the early 1980s, the headquarters of the Israel-Lebanon Mixed Armistice Commission (ILMAC) which was already located in Beirut, was also tasked with being the United Nations Liaison Office Beirut (UNLOB). Under the close supervision of UNTSO's Chief-of-Staff, UNLOB/ILMAC functioned as a dual purpose headquarters/ liaison office for both UNTSO and UNIFIL.