The South Lebanon conflict (1985–2000) or the Security Zone conflict in Lebanon refers to 15 years of warfare between the Lebanese Christian proxy militias SLA with military and logistic support of Israel Defense Forces against Lebanese Muslim guerrillas led by the Iranian-backed Hezbollah, within what was defined as the "Security Zone" in South Lebanon. It can also refer to the continuation of conflict in this region, beginning with the Palestine Liberation Organization (PLO) operations transfer to South Lebanon, following Black September in the Kingdom of Jordan. Historical tension between Palestinian refugees and Lebanese factions fomented the violent Lebanese internal political struggle between many different factions. In light of this, the South Lebanon conflict can be seen as a part of the Lebanese Civil War.
In earlier conflicts prior to the 1982 Israeli invasion, including Operation Litani, Israel attempted to eradicate PLO bases from Lebanon and support Christian Maronite militias. The 1982 invasion resulted in the PLO's departure from Lebanon. The creation of the Security Zone in South Lebanon benefited civilian Israelis, although at great cost to Palestinian and Lebanese civilians. Despite this Israeli success in eradicating PLO bases and its partial withdrawal in 1985, the Israeli invasion increased the severity of conflict with local Lebanese militias and resulted in the consolidation of several local Shia Muslim movements in Lebanon, including Hezbollah and Amal, from a previously unorganized guerrilla movement in the south. Over the years, military casualties of both sides grew higher, as both parties used more modern weaponry, and Hezbollah progressed in its tactics. By the early 1990s, Hezbollah, with support from Syria and Iran, emerged as the leading group and military power, monopolizing guerrilla activity in South Lebanon.
By the year 2000, following an election campaign promise, newly elected Prime Minister Ehud Barak withdrew Israeli forces from Southern Lebanon within the year, in accordance with UN Security Council Resolution 425, passed in 1978; the withdrawal consequently resulted in the immediate total collapse of the South Lebanon Army. The Lebanese government and Hezbollah still consider the withdrawal incomplete until Israel withdraws from Shebaa Farms. Following the withdrawal, Hezbollah has monopolized its military and civil control of the southern part of Lebanon.