FOR a week the Israeli air force attacked the militant Islamic group Hamas in Gaza in the hope of destroying its ability to launch rockets that have menaced Israeli cities. The strikes failed.

Despite the destruction wreaked on Gaza, Hamas was yesterday still able to fire off 30 missiles against Israel, some capable of reaching cities 25 miles from Gaza.

Hamas had been preparing for months for exactly the present conflict. A senior Hamas militant described last week how they had changed their rocket-launching operations to survive the expected Israeli air assault.

Previously Hamas military units would trundle vehicles to a site and fire off a rocket – a near-suicidal action if, as last week, Israeli drones, jets and helicopters were patrolling Gazan skies. Instead, the Hamas official said, his men had dug tunnels and run lengths of detonation wire so that they could launch from a distance.

The missiles, he said, were fired from makeshift frames or even car jacks, and replaced through the tunnel network. “We lose a tube or a framework worth $10, not soldiers,” the official said. Hamas members were ordered to swap their uniforms for civilian clothes and keep their guns tucked under their jackets.

The tolls of the conflict are disproportionate - 460 Palestinians have been killed and more than 2,100 wounded, while Hamas’s rockets have killed four Israelis. Nor has the Hamas barrage been anywhere near as effective as the onslaught by Hezbollah, an Islamic militant organisation in Lebanon, in the summer of 2006, when thousands of missiles hit Israel and the Israeli defence forces lost 119 soldiers.

Nevertheless, Hamas’s ability to keep launching rockets presented Israel with a serious dilemma. Should it call off the offensive in the face of increasing international pressure fuelled by civilian casualties? Or should it send tanks across the border in a last blitz aimed at destroying the Hamas infra-structure?

Last night Israel made its choice and sent tanks across the border into Gaza. After a bombardment by artillery and naval forces, armoured vehicles and troops moved in under cover of darkness. Helicopters were heard overhead and large explosions and flares lit up the skyline.

So how had Hamas, besieged for more than a year by the Israeli military, managed to procure an arsenal that put more than 750,000 Israelis under threat? And can the Israelis root out the weapons and Hamas leadership, who have developed networks of underground bunkers and tunnels all over Gaza?

Inside one of those underground networks Ahmed Ja’abari, 49, the military commander of Hamas, was preparing for the fight of his life.

Like all Hamas leaders, Ja’abari, a heavy-set man with a stubble beard and a preference for camouflage uniforms, had been living on the run since the Israeli onslaught began last Saturday, a day after a six-month ceasefire between Israel and Hamas expired.

And for good reason: on Thursday a one-ton bomb dropped on the home of Nizar Rayyan, a Hamas leader, killing him, his four wives and 10 of his 12 children. On Saturday another Hamas leader, Abu Zakaria al-Jamal, died in another airstrike.

By yesterday, 400 targets, including all Hamas’s offices and command and control centres, had been destroyed and hundreds of Hamas militants killed. Yet Hamas had still launched 430 rockets. How?

The answer goes back to Hamas’s increasingly sophisticated procurement programme and training assisted by Iran.

Hamas has about 15,000 militants in arms, a disciplined force that has built underground tunnels throughout Gaza, which is home to 1.5m Palestinians living in crowded, impoverished conditions. It has divided Gaza into six regions, each manned by a brigade. “If the Israelis attack us, they will be surprised by fighters who will appear from underground to fight them with advanced weapons,” said Jamal Jarah, known as Abu Obeida, a Hamas spokesman, before the ground assault began.

During the six-month truce, Hamas imported Russian-designed Grad long-range rockets from Iran through its cross-border tunnels and stepped up its home production of Qassam rockets, made in backstreet machine shops.

According to a senior Hamas commander, Hamas also began sending its elite troops to Tehran two years ago for training in field tactics and weapons technology. Many returned as instructors. “Everyone has been working overtime,” said the Hamas officer.

Israel knows the risks. “Even if we occupy parts of Gaza it will only be the ‘upper Gaza’, while ‘underground Gaza’ will still be fighting us,” said an Israeli officer. As always, civilians will also be victims.

In Ashdod, an Israeli port less than 30 miles south of Tel Aviv, Irit Shitrit, 39, a school teacher and mother of four, and her sister Ayelet were on their way back from a workout at the local gym when the sirens went off, signalling an incoming Hamas rocket.

Irit was driving and jumped out to find shelter in a nearby bus stop. “We heard the scream of a rocket,” said her sister, who survived the blast. “I held her close but all of a sudden I heard a big blast and Irit got a direct hit.” She was killed instantly.

In Gaza, the UN puts the civilian death toll at more than 100 but it is hard to distinguish which of the mounting corpses were fighters and which not.

The Balousha family lost five daughters in one night. “I was sleeping with my baby, Bara - she is only 12 days old,” said their mother, Samira Balousha, who could barely stop crying as she spoke. The two-room home of the impoverished family was next door to a mosque hit in the middle of the night by an Israeli missile. Her children Tahrir, 17, Ikram, 15, Samar, 12, Dina, 8, and Jawaher, 6, died. The baby survived.

Even before the current fighting, 70% of Gazans relied on UN food rations. International aid workers now fear a disaster. Yesterday Hatem Shurrab, who works in Gaza for Islamic Relief, a UK-based charity, said: “The hospitals were already low on supplies; they can barely cope now.”

The ground assault threatens to make matters much worse. Israel, however, seems determined to eradicate the threat from Hamas’ rockets, an aim apparently endorsed by George Bush, the outgoing US president. Yesterday he used his weekly address to condemn Hamas, saying: “Another one-way ceasefire that leads to rocket attacks on Israel is not acceptable.”

Tomorrow, Arab foreign ministers will urge the UN security council to call for a ceasefire. And this week Nicolas Sarkozy, the president of France, will arrive in Israel with proposals for a ceasefire.

Unfolding of a bloody conflict

December 27: Israeli jets launch airtrikes on Gaza, targeting Hamas leaders and installations.

December 28: Israeli aircraft bomb 40 smugglers’ tunnels serving the Gaza Strip, as well as buildings at the Islamic University.

December 29: Israel declares areas around Gaza “closed military zones” and targets a Palestinian Authority government building for the first time.

December 30: Israel continues attacks. Hamas fire rockets and mortars deep into southern Israel.

December 31: UN security council cannot agree on a ceasefire resolution. Some 40 Palestinians are killed in further air strikes, while rockets fired by Hamas strike Beersheba in Israel.

January 1: Nizar Rayyan, a hardline Hamas leader, is killed when an Israeli plane bombs his home.

January 2: Hamas calls for a “day of wrath” and 30 rockets are fired into Israel by militants.

January 3: 11 Palestinian civilians are killed in an airstrike on a mosque. Israeli troops advance into Gaza in conflict’s first ground action.

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