Israel's ground offensive is reaching a critical stage where its forces may soon have to face Hamas fighters on their chosen killing ground, the narrow, winding alleyways of Gaza City.
Despite the days of relentless air strikes, Israeli commanders admit that the Islamist movement still has large quantities of weapons and up to 20,000 trained men to use in a bloody campaign. The Hamas arsenal has been smuggled in through the intricate network of tunnels that dip under the Egyptian border, a network that has also provided the economic lifeblood for the Palestinian territory suffering from severe and punitive sanctions imposed by Israel.
The fact that these tunnels have played a key role in keeping Hamas in political and military power has made them not only targets of Israeli attacks but also a key issue in any ceasefire.
There are believed to be hundreds of tunnels criss-crossing the nine-mile wide barren border between Gaza and Egypt along what has become known as the Philadelphi Corridor. Constructed over years and varying in depth and width, the tunnels have carried everything from rockets to cattle. Some also allow access to routes for supposed VIPs to have quicker entry and exit from the Palestinian enclave.
Professor Efraim Inbar, director of Israel's Bar-Ilan University's Begin-Sadat Centre for Strategic Studies, believes the Israeli government bears a degree of responsibility for the existence of the tunnels. "They did not take them seriously at first and did not invest enough money and resources to detecting and stopping them. The problem they now face is that they have to destroy all the main tunnels and then also make sure that they are not rebuilt in the future."
In the past the Israelis have considered a number of options to deal with the tunnels, including digging a moat flooded with seawater, deterring smugglers with the risk of drowning. The plan was dropped, however, after it became apparent it could contaminate Gaza's crumbling underground aquifer. The Israelis have also asked the US to provide its Army Corps of Engineers to build an underground wall on the Egyptian side of the Philadelphi Corridor. The Americans are said to have agreed in principle although it is unclear whether the Egyptians had also given the green light.
Jerusalem's demand that there should be stringent checks carried out by an international force to monitor any ceasefire shows the Israeli anxiety about the underground routes being reopened. For the moment, the Israelis are trying to destroy the network with pulverising bunker-busting bombs acquired from the US. Nicholas Pelham, an analyst with the International Crisis Group, said closing the tunnels permanently would be a major and lengthy undertaking. "Without occupying a fairly broad stretch of territory, it is hard to see how you can maintain the closure of the tunnels long-term," he said.
The more immediate concern for Israel is how Hamas's armed wing, the Izz al-Din al-Qassam Brigades, have used the tunnels to prepare for battle. The militia is said to have sent thousands of members for training to Iran and Lebanon, where they have drawn on the tactics used successfully by Hizbollah against the Israelis in 2006.
According to one Hamas commander, one of the lessons learnt is to reduce the risk of taking return fire by not detonating missiles on site. Fighters "dig tunnels and use lengths of detonation wire so they can launch missiles from a distance. So we lose a tube or a firing frame worth $10, not soldiers".
Abu Bilal, a commander of Islamic Jihad, which operates independently from Hamas in Gaza, acknowledged the rocket attacks have been psychologically damaging for Israelis but have little military impact. "We can't do anything but fire the rockets and hope they enter Gaza," he said. "We are praying for the tanks to come so we can show them new things. All our fighters wait for the chance to kill them."
Hamas's weaponry includes Qassam missiles, mainly manufactured within Gaza, and Chinese copies of Russian-made Grads smuggled from across the border along with mortars, rocket-propelled grenade launchers, heavy calibre machine guns, mines and improvised devices. Hamas also inherited a stockpile of US-made small arms and ammunition abandoned by the rival Fatah movement when its fighters were driven out of Gaza in 2007.
Hamas has also upgraded its military structure with five brigades under separate commanders who report to Izz al-Din al-Qassam chiefs but have also been trained to carry on as individual units when necessary.
The militant group has also been beefing up its propaganda offensive to try to take on the sophisticated Israeli machine. With journalists barred from Gaza by the Israeli military, the Hamas website – the Palestinian Information Center (PIC) – is the main weapon in getting its side of the story to the wider world. It pumps out reports of heavy casualties among invading Israelis, emotive accounts of Gaza civilians "eradicated", and vows to strike deeper and harder into enemy territory.
The website yesterday displayed the mobility of a guerrilla fighter. Saying PIC was under "violent and organised electronic attack", Hamas engineers deftly offered another web address "in case of the halting of the site". Israel disrupted Hamas's al-Aqsa TV station, inserting a cartoon showing Hamas fighters being blown up coupled with an advisory "You won't succeed."
The website says Israel's ground operation amounts to "swimming in the blood of women and children". The PIC says Hamas is holding its own, inflicting at least 11 fatalities and dozens of injuries on Israeli troops. "The surprises are just beginning," it suggests. "Disciples ... are waiting for the Zionists with explosive belts" according to one article, while another spoke of the "Nazi occupation army".
What is Hamas? The origins and mission
Who are they?
In Arabic, the word "hamas" means zeal, but it is also the Arabic acronym for the Islamic Resistance Movement. The group came into being in 1987 after the eruption of the first intifada.
How is it organised?
The armed element, the Izz al-Din al-Qassam Brigades, carries out suicide bombings – the first was in 1993. The political wing, which romped to victory over Fatah in the 2006 elections, runs local government and services.
Who are the key people?
The de facto prime minister of Hamas is Ismail Haniyeh. The group's overall leader, Khaled Meshal, lives in exile in Syria. He was poisoned by Israeli secret services in Jordan in 1997, but King Hussein forced Israel to send an antidote to save his life.
What are Hamas's aims?
In the short term, Hamas wants to drive Israeli forces from the occupied territories. It is committed to the destruction of Israel and, in the long term, wants to establish an Islamic state on all of historic Palestine.
How is Hamas viewed?
A 2007 Pew survey found that almost two-thirds of Palestinians had a favourable opinion of the group. Iran and Syria both support Hamas, while all other Arab countries formally back the Palestinian Authority led by Mahmoud Abbas. The US and the EU have branded Hamas a terrorist organisation, but have sought an easing of the Israeli blockade of Gaza.