The event that was decisive in changing the nature of the invasion was a massacre of "Palestinian" Arab refugees at the Sabra and Shatila camps by a Christian Lebanese militia force, known as the Phalangists.
The fact that thousands of Lebanese Christians had been killed during the Lebanese civil war by "Palestinian" Arabs was ignored by the world's press; so was the fact that five thousand "Palestinian" rebels had been killed by General Zia ul Haq in Jordan in 1970. The fact that Security Minister Sharon was supposed to watch over the Phalangist forces for possible abuses was focused in on like a laser.
In the end, Israeli forces withdrew to the Litani and a zone south of it to prevent Arabs from firing missiles into Israel. But not before major internal damage had been done to the country's political structures and the trust that Israelis placed in the IDF.