Israel is determined to prevent the S-300s from becoming operational if they do arrive. The S-300s are a threat to Israeli aircraft and Israel will continue its air raids in Syria to stop any new weapons from getting to Lebanon and Hezbollah and to halt activation of the S-300. If the S-300 did show up in Syria (or Lebanon) Israel would probably attack it right away, before these systems could become operational. If Syria wanted to get the S-300s operational quickly they would need the help of the Russians, who would probably become casualties from the Israeli air attacks. The Russians might risk it because they have seen their weapons used on the losing (Arab) side in the Middle East for over four decades. Sure would be nice to turn this around. An attempt at this would tempt Russia to introduce more than a few troops and technicians to help activate the S-300 systems. Even then, the Russians would be up against more experienced and determined troops and risking another embarrassing defeat. This game of bluff has been played out in private by Russian and Israeli diplomats for years. The three Israeli air raids on Russian weapons in Syria this year were the Israeli response to Russians flying in more missiles (anti-ship and less capable anti-aircraft systems). The Russians keep changing their minds on the S-300s, which, if operational, can detect and attack aircraft 200 kilometers away, deep inside Israel.
Russia — and Syria — have more to lose here than Israel, because Israel can’t afford to quit until those missiles are gone. Another threat is if any S-300s should fall to any of Syria’s al Qaeda affiliates. I don’t know that they could actually successfully use the things without Russian assistance, but also don’t want to find out.