There are two countries fighting with the Assad regime that are responsible for its victory in Syria, namely Iran and Russia. Now that Assad again controls about two thirds of Syria, differences have appeared in the interests of these two countries. Both have their own interests in Syria, but neither want to jeopardize Assad’s resurgence by an outright clash. In order to prevent Iran taking over Syria the Russians are content to sit back and let Israel and others attack Iran’s expansion program in Syria. This reverses the usual adage, in this case the enemy of my friend is my friend.
Israel, also in return, has not opposed Putin’s drive to expand his influence in Syria and the Eastern Mediterranean. Although the US used to be the main power in the Middle East, both the Administrations of Obama and Trump have been retracting US forces from the region and the Sunni Arabs States and Israel have come to realize that there is a power vacuum created that both Russia and Iran seek to fill. But while Iran represents an existential threat to the survival of Israel and Saudi Arabia and the other Arab States, Russia does not. So the choice is easy. Fight Iran, but tolerate Russia.
Russia also gains significantly from conflict in the Middle East, since it pushes up the price of oil, and Russia is one of the major non-Middle East suppliers of oil in the world, and most of its import income comes from sales of oil. So Russia gains in both ways, it entrenches its position in Syria if Israel attacks Iran and it gains from the increase in the price of oil if Iran attacks Saudi Arabia. Not a bad deal.