As President Bashar Assad and his allies gain the upper hand, a new phase in Syria’s war has set Israel and Iran-allied militias on a collision course.Israeli officialdom agrees that the next war with Hezbollah, greatly empowered since 2006, will cause enormous harm at home. Residential towers in Tel Aviv will be struck, with many casualties, as will strategic sites like the Ben-Gurion Airport and the maritime gas rigs. That Lebanon would pay a much higher price may offer little consolation to Israelis in harm’s way.To prevent Syria from becoming a spark for Israel-Hezbollah or even Israel-Iran wars, there are two key realms to contend with: control of Syria’s southwest, and the establishment of Iranian and Hezbollah military- strategic assets across Syria.The southwest has been largely quiet because of the southern de-escalation zone established in Syria in May 2017 as part of the Astana deal and strengthened with a subsequent US-Russian-Jordanian agreement in November 2017. The de-escalation allowed Assad and his allies to focus on other parts of the country.Having essentially completed the retaking of the east, they have shifted their efforts elsewhere and other de-escalation agreements, in Idlib and eastern Ghouta, seem to be unraveling. The southwest might be next. Indeed, last month the Assad regime seized some territory in the zone’s northern edge, around Beit Jinn. Because the Syrian Army is probably unable to retake this area without the help of Hezbollah or other Shi’ite militias, further regime advances might trigger Israel to attack advancing Syrian Army units and militias in order to block Iran-backed forces from establishing offensive infrastructure in the area.Once the militias are in and around Quneitra, it is hard to see who will dislodge them if they want to stay.In parallel, growing Iranian efforts to establish a long-term military presence across Syria conflict with Israeli security interests. Israeli officialdom sees great risk with Iran building a seaport, airport, permanent military bases or high-precision missile factories, which would enable precise attacks on key Israeli facilities. Israeli strikes over the past months against such targets, and more aggressive Syrian reactions, indicate that the Syria war is already entering a new phase.Already, the parties are trying to establish new “rules of the game” through attack and response. But until those rules are clear, a wider war is only a miscalculation away.Redlines, however clearly one side might consider them, are operationally vague. Israeli officials have said that they won’t tolerate a “concentration” of militia forces in the southwest, but how many Hezbollah fighters have to be present before they become a “concentration?” Russia, an important backer of the Assad regime, is the only power in Syria in a position to broker two new sets of understandings to reduce the risk of a larger confrontation. First, Moscow should broker understandings that bolster the de-escalation agreement distancing Iran-backed forces from Syria’s armistice line with Israel. To do so, Moscow at the very least should not provide air cover for a regime campaign that includes Hezbollah or militias to retake these areas, and it should use its control of Syria’s airspace to prevent the Syrian Army from using its remaining air assets in the southwest. This alone would likely not suffice. Assad sees the opposition weakening as a result of the termination of US covert assistance and has declared he will retake the entire country.One formula to lessen the regime’s incentive to break the deal would be for rebels to hang on to their weapons for self-defense and policing and, in return, accept the state’s legitimacy (though not explicitly the regime’s) and the return of its administrative organs (staffed, to the extent possible, with local personnel).Israel would have to settle for a smaller exclusion zone than it has demanded, but this formula would offer a realistic way to keep its foes away and thus reduce the odds of a regional conflagration. Stability in the area could also facilitate a fuller return of the UN Disengagement Observer Force, an element of the 1974 Israeli-Syrian separation of forces agreement, which both Israel and Syria claim to want to restore.Perhaps the most significant challenge to this arrangement would be Russia’s ability and willingness to deliver the regime and Iran should it prefer not to clash with its partners and undermine their cooperation elsewhere. It is particularly unlikely that Damascus and Tehran would accede to this arrangement for the southwest so long as Israel continues to strike their assets in other parts of the country – which is why any local cease-fire is unlikely to endure without a broader modus vivendi.Second, Moscow should, therefore, seek to broker an Israeli-Iranian modus vivendi in Syria, one in which Iran waives construction of precision missile facilities and its military infrastructure in Syria and Israel acquiesces to foreign forces remaining in the rest of Syria pending a deal on the country’s future.To some in Israel, this arrangement would be a concession too many.But Russia is not only a constraint on Israel, it also could be of use as the only actor that has at least some leverage over Iran and its partners.Helping Israel avoid an all-out war that it does not want to fight would be no small service.Israelis, Lebanese and Syrians should all hope that the leadership in Moscow takes a proactive stance and wrestles seriously with the question of how to persuade Iran, and Israel, to pursue stability and prevent a new conflagration in Syria.The author is a senior analyst with the International Crisis Group’s Middle East Program.