“Sometimes you have to subordinate strategic considerations to tactical needs,” says Ehud Barak, Israel’s defence minister, former prime minister and the country’s most decorated military man. This is one such time: Mr Barak, backed by the current prime minister, Binyamin Netanyahu, is going to agree to Egypt deploying thousands of troops in Sinai even though the Israel-Egypt peace treaty strictly forbids it. They will have helicopters and armoured vehicles, Mr Barak says, but no tanks beyond the lone battalion already stationed there.

The decision comes after an audacious attack on August 18th on Israeli vehicles travelling on a scenic road that hugs the Israel-Egypt border and ends at the resort town of Eilat. Eight Israelis, civilians and soldiers, died in the attack and in shoot-outs involving the army. Ten attackers were killed, two apparently by Egyptian border guards, six of whom were also killed in the crossfire. Egypt blamed Israel for the deaths. Israel replied that a hard-core group of Palestinians, all heavily armed, had entered Egypt’s Sinai peninsula from Gaza a month ago, camped and trained there, and made their way unhindered across open desert to the site of the attack. Egyptian and Israeli security sources believe that several militants operating in Sinai joined them to take part in the attacks.

Israel responded immediately with an air strike on the southern Gaza town of Rafah, killing several leaders of the Popular Resistance Committees whom the Israelis accused of planning the attack. Palestinian militants hit back with missile salvoes, killing one Israeli in the town of Beersheba. Further air strikes destroyed would-be missile launchers, according to the Israelis, but also killed three children, according to Palestinian medical sources.

Israeli-Palestinian tit-for-tats have become all too familiar. But the Egyptian involvement is new and has quickly developed into a political crisis. Angry demonstrators surrounded the Israeli embassy in Cairo. A young handyman, Ahmed al-Shahhat, now known as “Flagman”, scaled the building and replaced the Israeli flag flying there with an Egyptian one. The interim military government hinted at one point that it was recalling its ambassador, but later seemed to have changed its mind. Mr Barak and President Shimon Peres offered apologies for the killings, but Egypt said these were insufficient. An Israeli general, sent discreetly to Cairo, proposed a joint Israeli-Egyptian inquiry into the deaths of the Egyptian border guards.

Israel faces a dilemma with far-reaching strategic consequences. Thirty years of peace with Egypt have rested, above all, on a demilitarised Sinai. The peninsula is patrolled by an international force and monitored by America from the air, to ensure that both sides keep their armies out, even though Sinai is sovereign Egyptian soil. Until now, Israel had said no to Egyptian demands to let more troops on to the peninsula, beyond what is specified in the 1979 peace treaty. Yet it urgently needs Egypt to tighten security. “If nothing is done today,” an aide to the Israeli prime minister says, “you will see extremist groups establishing a larger footprint in Sinai.”

Egypt has for years had difficulty imposing order on the peninsula. The situation has worsened since the toppling of Hosni Mubarak, the Egyptian president, in February. At the end of July, dozens of gunmen attacked a police station in el-Arish, Sinai’s biggest city. A mixture of banditry, tribal infighting and jihadist activity means the authorities “have little control beyond the city,” says an el-Arish resident. Criminals—some of whom escaped from prison during Egypt’s revolution—have blocked roads and carried out numerous carjackings. Jihadist groups (sometimes claiming to be “al-Qaeda in the Sinai peninsula”) have called for the creation of an Islamic emirate.

The sudden change of power in Cairo has accelerated the collapse of central authority in Sinai. It has also given freer voice to a widely felt animosity towards Israel among the Egyptian public, a sentiment which the Mubarak government kept carefully muffled.

Mr Barak does not downplay Israel’s long-term concern or the risk in what he is proposing. The new troops allowed into Sinai are unlikely ever to be withdrawn by any Egyptian government. In themselves, the few thousand men in question will not pose a serious threat. But Sinai was an Israeli-Egyptian battlefield in four bitter wars. Troop movements there have tended in the past to generate pernicious dynamics of their own.