Unless things have changed, Russia has or had 50,000 troops in Syria defending the regime from ISIS and other internal (and external terror) threats. Assad Jr. can’t count on his own internal security forces and the Assads come from an ethnic minority that rule over the majority in Syria. While I don’t care much for Assad, Israel and Russia coordinate on attacks against these terrorists who would use Syria as a base of operations against Assad and Israel. I don’t see Israel giving much assistance to Ukraine, other than humanitarian. Sure, Zalensky may be Jewish but he’s not Israeli.
The reason Russia was there is they have a naval base there.
They got a little concerned that if Assads regime failed, they would lose that.