Will this be the eighth war between Israel and Hamas?

And so it begins again. Seven times since the foundation of the state of Israel 64 years ago the country has found itself at war, sometimes on a front of its own choosing, sometimes in a setting imposed by its enemies. The new conflagration in the Gaza Strip may go down in history as war number eight. Once again, the condensation trails of jet fighters are streaking across a perfectly blue Mediterranean sky, while explosions echo over the white tower blocks of Gaza City and the small, neat houses of southern Israel.

Another pattern is familiar: this conflict has caught the world by surprise, just as the outbreak of the last Gaza war in 2008 and the Lebanon campaign of 2006 were both unexpected events, at least for outsiders.

So why is this happening now? And what dilemmas will confront Israel and Hamas, the radical Islamist movement, as they wage the latest round of their struggle? Like circling scorpions, these two foes watch each other obsessively. Every move they make sends a signal - and both sides have tacitly established a set of rules. Israel interpreted the latest decisions by Hamas as breaking the implicit code and sending a clear message of escalation.

Two events in particular are known to have influenced Israel's decision to launch its offensive. Last week, it discovered an ambitious new tunnel starting in Gaza. In itself, this was nothing unusual: a web of tunnels fanning out beneath the border with Egypt has supplied the Palestinian enclave ever since an Israeli blockade throttled the inflow of almost everything except humanitarian essentials. But this partially complete artery did not point towards Egypt: it was burrowing under Gaza's other frontier, beneath Israel itself.

That was strike one - and it summoned bitter memories. In 2006, a tunnel of this kind - which Hamas had managed to build undetected - was used to mount a remarkably effective raid on Israeli territory, causing the deaths of two soldiers and the abduction of Corporal Gilad Shalit, who spent the next five years in captivity in Gaza.

Israel duly destroyed the tunnel and, unusually, sent its troops several hundred yards inside Gaza to do the job. Last Saturday, Hamas operatives then fired an anti?tank missile into a military jeep as it drove along a road inside Israel, east of the Gaza border. All four soldiers inside were wounded, one of them critically.

Instead of firing desultory barrages of rockets at southern Israel, most of which explode harmlessly in open fields, Hamas had struck a military target within its neighbour - and only luck had prevented lives from being lost. That was strike two.

Israel put the two events together and decided that Hamas had chosen escalation. Whenever this happens, the country's securocrats warn that their power to deter Hamas has also waned. Israeli military doctrine is based on the "power of deterrence", namely the country's ability to pound its enemies so effectively that they refrain from doing any damage.

That will have been the rationale for the air strikes that began on Tuesday and, in particular, for the opening blow that killed Ahmed al-Jaabari, the commander of Hamas's military wing.

Hamas, for its part, will see things differently. In Hamas's interpretation, the Israeli attack on the tunnel was the key escalation - and the destruction of the military jeep a proportionate act of retaliation. Meanwhile, the assassination of its military chief on Tuesday will be seen by Hamas as the clearest way of inflaming the situation, hence it struck back with the rockets that killed three Israeli civilians yesterday.

So the question "why now" has a straightforward answer: now happens to be the time when a combination of events, none of them particularly important in isolation, has convinced each side that the other is determined to escalate. And so both enemies have chosen to land their heaviest blows.

Benjamin Netanyahu, the Israeli prime minister, is no doubt thinking of the election due on January 22. Now is a particularly bad time to seem indifferent to Israelis who must live within range of rockets fired from Gaza. In the aftermath of the US election, moreover, there is a changing of the guard in Washington and the impending arrival of a new secretary of state. Netanyahu might believe this temporary power vacuum gives him more freedom of manoeuvre.

But Israel is also haunted by the memory of its withdrawal from south Lebanon in 2000, when Hizbollah, the Shia extremist movement, soon came to dominate the frontier. Hamas's actions in Gaza were beginning to "replicate what happened in Lebanon when Israel gradually ceded control of the border area to Hizbollah," says David Horovitz, editor of The Times of Israel, a news website. "Hamas was starting to build the capability to deter Israel's options for acting in Gaza."

On Horovitz's analysis, the Israeli government had to act before Hamas built enough tunnels and amassed enough weaponry to establish its own "power of deterrence". The next big decision will be whether to follow up the air strikes with a ground assault. Sending Israeli troops into Gaza would hugely increase the risk of casualties on both sides, without necessarily achieving any military objectives. If the aim is to destroy weapons and kill key Hamas operatives, then that can be done from the air.

General Danny Yatom, a former head of the Mossad, Israel's external intelligence agency, urges the government to be cautious. "I hate the idea that we will have to enter by land into Gaza," he says. "It is a highly populated area. The manoeuvrability of ground forces is greatly limited under those conditions.

"Such an operation might cause on our side many more casualties than would be caused by rocket attacks and, in addition, there is no doubt it would cost many more lives on the Palestinian side. I think a ground operation is needless, unless there is a deterioration."

Yet no military operation can turn Gaza into a stable and harmless neighbour for Israel. In the absence of a peace settlement, and with Hamas firmly in command, Gaza will remain an impoverished enclave of misery and extremism. About 1.6 million Palestinians are crammed inside this territory; some of its refugee camps are among the most densely populated places on earth. The Israeli blockade has helped to strangle the economy, reducing about 80 per cent of Gaza's population to dependence on humanitarian aid.

And this lamentable situation can only worsen: Gaza's population doubles every 25 years. At some point this barren, overcrowded, war-torn patch of land will cease to be a viable place of habitation. One United Nations forecast suggests that Gaza will be "unliveable" by 2020.

"The problem of the Israelis is that they look at Gaza only from the angle of security and military," says Sufian Abu Zaydeh, from the Palestinian Liberation Organisation negotiating department. "The problem of Gaza is political, it's humanitarian and it's economic. I always say to Israelis, 'If you are not allowing people in Gaza to live a normal life, don't ask them to think and behave in a normal way.'?"

As for the popular reaction to Israel's offensive, Abu Zaydeh, an opponent of Hamas, said: "I think people in Gaza will become more in solidarity with Hamas, rather than looking to Hamas as responsible for the tragedy."

The state of Gaza reflects the moral bankruptcy of both sides: of Hamas, whose only idea for governing the territory is to use it as a giant launch pad for firing rockets at neighbouring civilians, and of successive Israeli governments, who see the territory only as a security problem to be corralled and, on occasion, bombarded.

Senior Israelis sometimes say in private that instead of trying to solve their conflict, the world should learn to live with the status quo. As civilians die on both sides, the human cost of that status quo is being driven home once again.