From pg. 382, A History of the Modern Middle East by Martin Bunton and William Cleveland
"The [Lebanese Civil War] was expanded and then brought temporarily to an end by Syrian intervention. In May 1976 President al-Asad of Syria sent his army into Lebanon to rescue the Christian militias from the battering they were taking at the hands of the PLO and the forces of Jumblatt. Al-Asad’s choice of allies was perplexing because it created a situation in which Syrian troops and Maronite militiamen fought side by side against the PLO and the forces of the National Movement. Whatever al-Asad’s motives may have been for supporting the Maronite faction, Syria’s invasion of Lebanon escalated the fighting and expanded the level of destruction."
As noted, this is very perplexing. Syria has always had an extremely anti-Israel position, never having formally recognized the country. Wouldn't Syria supporting the Christian Maronites and fighting the PLO be strategically beneficial for Israel? Indeed, destroying the PLO in Lebanon (thus getting a free hand in the West Bank) and installing a Christian government were Israel's principal objectives when it invaded Lebanon in 1982.