While it is hard to morally justify Hafez al-Assad’s intervention in Lebanon in 1976 on the side of right-wing Christian militias, it is important to understand his reasons for soing so at the time were not to serve Israeli interests.
The explanation is found in this classified Pentagon document Donald Rumsfeld recently released and published in Akhbar Arabic here:
"He is, in sum, a prudent and careful, but tough, practitioner of statecraft.
Several military factors have operated to reinforce his natural tendencies and cause him to opt for a course or action which seems antipathetic to the image of revolutionary Ba’athist Syria…
Given the hostility of his eastern neighbor, Asad is loath to see emerge on his western flank a radical leftist- and Palestinian dominated Lebanon, almost certainly unamenable to his direction. Furthermore, a radical Lebanon could drag Asad into a war with Israel at a time, place, and in circumstances not of his own choosing.
Moreover, a radicalized Lebanon would be a military liability as a confrontation state with Israel. Lebanon may never be able to field a credible military force against Israel and certainly could not do so for Lebanese-Israeli border, a mission for which they are clearly inadequate, or to present Israel with a virtually undefended corridor through which the IDF could outflank his forces on the Golan Heights.
Syria’s disinclination to have a radical Lebanon on its flank is a continuation of its traditionally rigid control of Palestinians in Syria. While it is extremely wary of the more radical “rejectionist” fedayeen groups, Syria clearly does not trust even the “establishment” PLO, which is—an undisciplined and shifting alliance often, as now, in disarray and impossible to regulate.”