On 11 March 1978, a PLO guerilla raid from Lebanon led to the Coastal Road Massacre. <answer>Israel</answer> responded by launching an invasion of southern Lebanon to destroy the PLO bases south of the Litani River. Most PLO fighters withdrew, but <answer>Israel</answer> was able to secure southern Lebanon until a UN force and the Lebanese army could take over. The PLO soon resumed its policy of attacks against <answer>Israel</answer>. In the next few years, the PLO infiltrated the south and kept up a sporadic shelling across the border. <answer>Israel</answer> carried out numerous retaliatory attacks by air and on the ground.
Who secured southern Lebanon?
Israel