IN THE early hours of April 9th Israeli fighter jets crossed into Lebanese airspace and fired a salvo of cruise missiles eastward. Their target was the T-4 military airbase in central Syria (see map on previous page), not far from the ancient city of Palmyra. More specifically, the missiles were aimed at a hangar in a secluded compound on the west side of the airfield. The building was used by Iran’s expeditionary Quds Force, the foreign wing of its Islamic Revolutionary Guards Corps (IRGC). Russia and Syria both claimed that they detected the Israeli aircraft and the incoming missiles, and that at least some of them were intercepted. But enough of them got through to cause significant damage and kill at least seven Iranian officers, including a colonel. The Israeli government has not publicly acknowledged that it was behind the attack.

Since the start of the Syrian war in 2011, Israel has carried out at least 100 cross-border strikes. Most were aimed at weapons convoys and depots belonging to Hizbullah, the Iranian-backed Lebanese Shia movement. The latest strike is only the third in which Iranian personnel were directly targeted. The last time Israel hit them was two months earlier, at the same base, after an Iranian drone breached Israel’s airspace. But this strike appeared to be pre-emptive, not reactive.

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For decades Israel has fought a shadow war with Iran, which funds Lebanese and Palestinian proxies that attack the Jewish state. In recent months, though, the war has escalated: Israel and Iran have come into direct confrontation on Syrian soil, where the IRGC is determined to establish permanent bases. To do so, it must find a way to limit Israel’s freedom to carry out air strikes there. Its officers at T-4 were working to build up an air-defence capability meant to threaten Israeli aircraft—which is why they were targeted.

Binyamin Netanyahu, the Israeli prime minister (pictured right), has placed great stock in his personal relationship with Russia’s president, Vladimir Putin (left). They meet and talk frequently. But Israel’s security chiefs are coming to realise that the Kremlin will not exert itself to limit Iran’s role in Syria. Mr Putin has committed himself to the survival of Bashar al-Assad’s regime. His air force provides it with overwhelming firepower, though for domestic political reasons he is reluctant to put Russian boots on the ground.

Instead the Assad regime has turned to Shia militias, often made up of Afghans drafted and organised by Iran, as cannon fodder. Some of the toughest battles in Syria were fought by Hizbullah. “Putin respects Netanyahu and Israel’s military power, and would prefer to be co-ordinated with Israel,” says an Israeli spook. “But to safeguard his interests in Syria, he needs Iran more right now.”

Russia and Israel have a military “deconfliction” process that, over the last two and a half years, has kept the countries’ aircraft from clashing over Syria. Russia has turned a blind eye to Israel’s frequent air strikes. But after the latest attack, Israel’s ambassador to Moscow was summoned to explain it.

In the past Israel has assured Russia that it does not seek to harm the Assad regime, as long as its own strategic interests in Syria are not jeopardised. That line is starting to change. Yoav Galant, the housing minister and a former general, has made a round of interviews calling for Mr Assad’s removal. While this is not yet official Israeli policy, Mr Galant is close to Mr Netanyahu. It is hard not to interpret this as a message to Moscow. Israel is determined to prevent Iran from expanding its foothold in Syria, even if it means threatening Russia’s client in Damascus in the process.