Even top-end estimates of casualties are but a fraction of the strength of Hamas' military brigades and other groups

The last time Daoud saw his son was when the 24-year-old joined his family to break the Ramadan fast in their overcrowded apartment in Rafah, south Gaza.

"He stayed for an hour or two. He was killed later during the night … I learned about it from neighbours," Daoud, a steel worker, said.

The son, Ibrahim, was a fighter with the Izz ad-Din al-Qassam brigades, the military wing of Hamas. A graduate in education who had joined the police, he died with five others in an Israeli airstrike on 8 July. The corrugated iron fence outside his home has been daubed with a verse from the Qur'an, asking God to accept his martyrdom.

As war returned to Gaza on Friday, many outside observers were asking why Hamas and the other smaller Islamist groups active in Gaza had let the ceasefire lapse and fired dozens of rockets into Israel. One answer lies in the resilience of a movement that has been carefully built over decades and is deeply embedded in the community.

A key question is the level of casualties sustained by Hamas, an acronym for the Harakat al Muqawamma Islamiya (Islamic Resistance Movement) so far in this most recent war. Israeli military officials have said that up to 900 fighters from Hamas and other smaller factions in Gaza, such as Palestinian Islamic Jihad, have been killed. These figures are largely based on reports from Israeli units fighting in Gaza.

"They were reporting a lot of success: five guys, 10 guys, 20 guys killed. These were big engagements," said Daniel Nisman, an Israeli security analyst and commentator.Last week, Yossi Kuperwasser, the Israeli minister for strategic affairs, told reporters: "Hamas has lost more and more during the war – the tunnels, thousands of rockets and hundreds of operatives. Every day that went by, they lost more."

It does appear that dozens of sophisticated tunnels leading from Gaza into Israel, which could enable cross-border raids to kill or kidnap civilians and soldiers, have been destroyed. More than 3,000 rockets have been fired on Israel from Gaza – killing three people – which Israeli officials insist is at least half of Hamas's total stocks of the weapons.

However, few senior Hamas military commanders appear to have died.

"Most of the casualties were from anti-tank missile cells or lost on motorbikes. These were low-level guys bouncing between missile positions, particularly in border areas," said Daniel Nisman, an Israeli security analyst.

When last week a missile struck a street metres from the gates of a UN school in Rafah, killing nine people, Israeli military spokesmen said they had been targeting "terrorists on a motorbike" nearby.

Khaleel Habeel, an Islamic Jihad official in Gaza, admitted casualties, saying that "if you take on the fourth most powerful army in the world then of course you lose people". Ziad Abu Oda of the Mujahideen Faction splinter group told the Guardian that his organisation had lost 50 men, including fighters and political officials.

But even top-end estimates of casualties would be a fraction of the strength of Hamas's military brigades and other groups, which are believed to have 10,000 fighters permanently under arms, with another 10,000 in reserve.

The intensity of the fighting also surprised Israeli commanders, with Hamas deploying powerful sniper rifles and using tunnels and a range of new tactics to kill 64 Israeli soldiers.

In Khuzaar, a border village badly damaged in intensive fighting, another father, who did not want to be named, described how his son and six other local men had hidden in Khuzaar to ambush Israeli troops. All seven were killed.Hamas, which was founded by Islamist ideologues in Gaza more than 25 years ago and is linked to the Muslim Brotherhood movement, has run orphanages, clinics, schools and youth activities through its social wing. These aim both to recruit for the military brigades and consolidate the group's broader support base in Gaza.

The aims of the political wing too are subordinate to those of the organisation. "If it is a choice between government and the movement, the movement is always the priority," said Omar Shaban, a Gaza-based analyst.

Shaban said the group's insistence that Israel eases, or entirely raises, the eight-year blockade of Gaza showed Hamas was in touch with the people. The blockade has crippled the economy, sending unemployment soaring and incomes plunging.

People in Gaza expressed almost unanimous support for the decision to return to hostilities on Friday, despite the deaths of about 1,800 people, mainly civilians. Many, though, are undoubtedly afraid to criticise the group openly.told his congregation that the community would emerge stronger from the conflict.

Not far away, around thirty children played and five adults slumbered through the hot hours of the afternoon in the basement of another overcrowded, crumbling building. Three of the four sons of Mohamed Ahmed Abdulhadi Hamed, 75, had been killed in an airstrike on their home in the town of Beit Hanoun four weeks ago. Eighteen of the children, including 11 boys, are now orphans and homeless.

Hamed admitted that one of his sons, 38-year-old Hafiz, had held a senior rank in Palestinian Islamic Jihad, perhaps the only faction to lose a significant number of senior commanders in the conflict so far.

"What do you think these boys will do when they become men?" Hamed asked. "Of course this will push these children to join the resistance. If someone kills your father do you leave him alone?"