Unlike the confused and improvised Israeli response as the war against Hezbollah in Lebanon unfolded in 2006, Operation Cast Lead appears to have been carefully prepared over a long period.

Israeli media reports, by usually well-informed correspondents and analysts, alluded on Sunday to six months of intelligence-gathering to pinpoint Hamas targets including bases, weapon silos, training camps and the homes of senior officials. The Cabinet spent five hours discussing the plan in detail on December 19 and left the timing up to Ehud Olmert, the caretaker prime minister, and his defense minister Ehud Barak. Preparations involved disinformation and deception which kept Israel's media in the dark. According to Ha'aretz, that also lulled Hamas into a sense of false security and allowed the initial aerial onslaught to achieve tactical surprise - and kill many of the more than 300 victims counted so far.

Friday's decision to allow food, fuel and humanitarian supplies into besieged Gaza - ostensibly a gesture in the face of international pressure to relieve the ongoing blockade - was part of this. So was Thursday's visit to Cairo by Tzipi Livni, Israel's foreign minister, to brief Egyptian officials. The final decision was reportedly made on Friday morning.

Why now?

Barak said on Sunday the timing of the operation was dictated by Israel's patience simply "having running out" in the face of renewed rocket and mortar attacks from Gaza into Israel when the shaky six-month ceasefire expired 10 days ago. "Any other sovereign nation would do the same," is the official Israeli refrain.

Amid the storm of international criticism of Israel's hugely disproportionate response, it is easy to overlook the domestic pressure faced by the Israeli government over its handling of "Hamastan".

Homemade Qassam rockets and mortars rarely kill but they do terrify and have undermined Israel's deterrent power as well as keeping 250,000 residents of the south of the country in permanent danger.

But the context now is February's Israeli elections. The contest that matters is between Livni's centrist Kadima party and the rightwing Likud under Binyamin Netanyahu, who talks only of "economic peace" with the Palestinians and does not want an independent Palestinian state, as Livni does.

Opinion polls show that it pays to talk tough: Livni's standing has improved in recent days. The US political timetable may be as significant. The three weeks before Barack Obama's inauguration were Israel's last chance to assume automatic diplomatic support from Washington, as it got from George W. Bush over both West Bank settlements and the Lebanon War.

It is hard to imagine an Israeli government testing Obama, whom it views with foreboding because of a sense he has more sympathy for the Palestinians, with a crisis of these dimensions during his first days or weeks in office.

Game plan

Livni and other Israeli officials have spoken openly of wishing to topple Hamas since the Islamist movement took over from the Western-backed, Fatah-dominated Palestinian Authority (PA) in June 2007.

But this may be something less ambitious. "The realistic objective of any military operation is not the ousting of Hamas, but rather ... undermining its military effectiveness and weakening its rule," is the view of Yediot Aharonot analyst Alex Fishman.

Ron Ben-Yishai, another military expert, called it an attempt to "change the rules of the game." This appears to be a case of "asymmetric warfare" in which the weaker party commands disproportionate force - by repeatedly firing crude rockets or using suicide bombers - and the more powerful one responds with a massive, disproportionate blow. "The objective of an Israeli military operation in Gaza must be to undermine Hamas' desire to keep fighting, and at that point agree on a ceasefire," said Fishman.

Israel is well-informed about what happens in Gaza. Its premise is that Hamas is unpopular and that by targeting its personnel it can encourage that trend. But not all the victims are from Hamas. Some are civilians and there are security officers who belong to Fatah. And nor, crucially, has the PA been able to deliver a peace agreement with Israel, or even end its settlement activity.

Most significantly, the scale of the bloodshed - ranking in Palestinian history alongside the 1948 Deir Yassin killings or the Sabra and Shatila massacres (by Israel's Christian Lebanese allies) in 1982 means renewed motives for hatred and revenge.

What next?

Israel said on Sunday that it is calling up thousands of reservists. There can be little doubt that it could reoccupy and hold the coastal strip - as it did from 1967 to 2005 - but tanks and infantry would be vulnerable in guerrilla warfare against lightly-armed but highly-motivated Hamas or Islamic Jihad fighters. Civilian casualties would grow with international pressure. The only reason to deploy ground forces would be to achieve something air power could not - searching for rocket production and storage facilities that have not yet been identified.

Israeli commentators suggest the army has no appetite for a ground war, making comparisons with Lebanon in 2006, and pointing to the impending elections.

Another key question for the military must be the fate of Gilad Shalit, the Israeli corporal held in Gaza since he was captured in 2006. It is hard to see negotiations on his release, and of Palestinian prisoners held by Israel, continuing in these circumstances.

The Gaza offensive has already fuelled anti-Israeli and anti-American feeling across the Arab world. Mahmoud Abbas, the Palestinian president, faces demands for an end to any talks with Israel. Hamas, calling for a "third intifada," accused Egypt and Jordan of colluding with the Gaza plan. If there is a silver lining in this dark cloud it is to have shown that working to achieve a lasting peace in the Middle East is still a desperately urgent task.