Syria gasses its own citizens.
After (too) much deliberation, the international community (this time, the US) decides that’s some kind of red line. (Just bombing and shooting tens of thousands of your citizens to death: okey dokey. Gassing them to death: hemming, hawing…we think this might not be ok.)
As such, the US may or may not attack Syria at some point.
Syria’s response: In that case, we’re gonna attack Israel.