In 1976, Syrian President Hafez al-Assad sent troops into Lebanon to fight PLO forces on behalf of Christian militias. This led to escalated fighting until a cease-fire agreement later that year that allowed for the stationing of Syrian troops within Lebanon. The Syrian presence in Lebanon quickly changed sides; soon after they entered Lebanon they had flip-flopped and began to fight the Christian nationalists in Lebanon they allegedly entered the country to protect. The Kateab Party and the Lebanese Forces under Bachir Gemayel strongly resisted the Syrians in Lebanon. Following the Israeli invasion into Lebanon in 1982, new Lebanese President Amine Gemayel sought the support of Syrian troops to stabilize the region. Syria fought Israeli troops after the latter's invasion of Lebanon.
Source:
https://en.wikipedia.org/wiki/Lebanon%E2%80%93Syria_relations#Lebanese_Civil_War_and_the_Syrian_occupation_of_Lebanon