“Since this morning, there has been an increase in the internal fighting in Syria and the Syrian Air Force’s activity,” the IDF said. “The IDF is in high alert and will continue to operate against the violation of the 1974 Separation of Forces Agreement.”

Under that agreement, which was signed by Syria and Israel in the wake of the 1973 Yom Kippur War between Israel and Arab states including Syria, the two countries agreed to a cease-fire on land, sea, and air, as well as to “refrain from all military actions against each other.” The accord has, for the most part, lasted since that time, albeit with exceptions sparked by the Syrian Civil War raging on Israel’s border. The last time Israel shot down a Syrian jet in its airspace was 2014. Syrian military drones and unmanned aerial vehicles have entered Israel and Israel has struck targets inside Syria since the civil war began in 2011.

Why Israel fears Iran’s presence in Syria

Israel’s action comes a day after Sergei Lavrov, the Russian foreign minister, visited the country with an offer that he said would keep Iranian forces in Syria at least 62 miles from Syria’s border with Israel. But, as Haaretz reported, Israeli officials are concerned that even with a 62-mile buffer zone, Iran can use its long-range missiles against Israeli targets.

“The removal of Iran must include the removal of long-range weapons, halting the production of precision weapons as well as the air defenses that protect the missiles, and the closure of border crossing that permit smuggling of this weaponry to Lebanon and to Syria,” an Israeli official told the newspaper. “Russia has a certain ability to prevent this. They are a significant factor in Syria.”

News reports suggested U.S. President Trump and Vladimir Putin, the Russian president, would reveal some sort of “grand bargain” on Syria at their joint news conference last week in Helsinki, an event that garnered attention for far different reasons. Under that reported “grand bargain,” the U.S. would accept Assad’s presidency in exchange for a full Iranian withdrawal from Syria. No such plan was announced in Helsinki.

Both Iranian and Russian forces are supporting Syrian President Bashar al-Assad in his country’s more than seven-year-long civil war. Assad has all but won that conflict, which has cost the lives of hundreds of thousands of civilians, created a refugee crisis that is upending politics far beyond the region, and destroyed the infrastructure of a country that was until recently a regional power. Many of the rebel groups, which were nominally supported by regional and Western powers, including the U.S., are now in talks with the Assad regime. ISIS, which once controlled a large swath of territory straddling Syria and Iraq, is now reduced to small pockets in Syria.

Israel has watched much of this with increasing alarm. It has stayed away from the fighting unless directly provoked, but has engaged in humanitarian actions, including providing medical care to injured Syrian refugees, and, last week, evacuating members of the White Helmets, the anti-Assad humanitarian group, and others from southern Syria to Jordan, via Israel. As it becomes increasingly clear that Assad, whose political survival was until recently in grave doubt, will remain in power for the foreseeable future, Israeli officials say they fear that his victory will ensure a permanent Iranian and Hezbollah military presence on its border in southern Syria; Hezbollah serves as Iran’s proxy. Israel, which says it views Iran as an existential threat and fought a long conflict with Hezbollah in southern Lebanon, is adamantly opposed to any such presence. Israel fears this would be tantamount to turning Syria into another Lebanon, where Iran enjoys influence through Hezbollah and from where it maintains the ability to strike targets inside Israel. Iran, meanwhile, is keen to preserve the rewards of its investment in the Syrian conflict.