Comment by nindalf

From the first two sentences on wikipedia

> The 1982 Lebanon War began on 6 June 1982, when Israel invaded Lebanon. The invasion followed a series of attacks and counter-attacks between the Palestine Liberation Organization (PLO) operating in southern Lebanon and the Israeli military that had caused civilian casualties on both sides of the border.

You asked "but what pissed them off so much". Maybe it was the PLO operating in Lebanon, not sure.