The US intervened 11 times over the last century in foreign civil wars: Korea (1950’s); Laos, Vietnam, Cambodia and the Dominican Republic (1960’s); Lebanon (1980’s); the Yugoslav Wars/Kosovo, Haiti, Somalia and Iraq (1990’s); and Libya (2011).
The success of these interventions, difficult as they are to assess, is mixed at best with Korea and Kosovo as examples of where positive outcomes occurred (in Haiti, a descent into complete chaos was perhaps prevented). Some turned out worse, given what followed.
As JPMorgan's Michael Cembalest notes, on a purely humanitarian basis, Syria’s tragedy is exceeded by many conflicts that the US abstained from participating in. So when thinking about civil wars and how the US defines its national interest, one has to ask why Syria would qualify for direct intervention while others conflicts did not.