The wife and child of Hamas military commander Mohammed Deif were killed in an Israeli airstrike after a sharp escalation in Gaza attacks. WSJ's Nick Casey gives us a closer look inside Hamas's secretive guerrilla army, the Qassam Brigade, on the News Hub with Simon Constable. Photo: Getty
DEIR AL-BALAH, Gaza Strip—When the shrapnel-torn body of Ahmed Abu Thoraya returned to this city in the Gaza Strip, only one member of his family knew for sure he had been a fighter in Al Qassam Brigade, the armed wing of Hamas.
Mr. Abu Thoraya had given his brother, Mohammed, a short will before he left town on July 19. "He said 'I'm going somewhere,' " his brother recalled recently. "I knew that he may not come back."
The conflict in the Gaza Strip has brought the secretive guerrilla army of Hamas out of the shadows and into battle against Israel's military for only the second time. When the brigade's fighters are killed, Hamas street organizers eulogize them as heroes, posting images of them in fatigues and toting rockets. And families in the Gaza Strip are coming to terms with never-before-discussed identities of sons and neighbors.
The fighting has given Israel its first good look at Hamas's street-fighting abilities since 2009—the only other time the Israeli Defense Forces have taken on large numbers of the Qassam fighters at close quarters. The Hamas militia has inflicted the heaviest death toll on Israel's military in a decade, some 64 soldiers so far. Israel and the U.S. regard Hamas, which also has a political wing and delivers social services, as a terrorist enterprise.
On Tuesday, the latest cease-fire broke down when a salvo of rockets from the Gaza Strip landed in southern Israel, and Israel retaliated against militant targets in Gaza. Truce talks in Cairo were suspended.
"Hamas has advanced on all fronts," said a senior official in the Israel Defense Forces. "This time when we meet them on the battlefield, they are better trained, better organized, better disciplined."

Hamas's internal communications proved more difficult for Israel to track, and Hamas exhibited a new capacity for aerial observation of Israeli troop movements. Hamas rockets, though mostly intercepted above Israel, managed to shut down Israel's main airport for a time.
This time, Hamas surprised Israeli soldiers by using a network of tunnels under the walls and fences enclosing the Gaza Strip to emerge inside Israel. Hamas commando units that Israel believes took shape mostly in the last year carried out complex ambushes inside and outside Gaza.That wasn't the Hamas that Israel encountered in its 2009 ground invasion of Gaza. When Israel's military entered the strip back then, Hamas fighters, for the most part, quickly melted away.
A 22-year-old Israeli infantry soldier said recently that Hamas units inside Gaza didn't wait for Israeli ground forces to enter the territory but instead fired mortars into military staging areas along the border inside Israel. In one incident last month, Hamas mortars killed four Israeli troops there and injured another four, the soldier said. "The danger on the border was just as bad as going inside," said the soldier.
In Deir Al-Balah, word emerged on July 19 that 14 of its men had been killed when an Israeli fighter jet dropped a bomb onto a group of Hamas fighters east of the city. Townspeople said the fighters had been part of a unit known as the Deir Al-Balah Brigade.
Deir Al-Balah, whose name refers to the date trees growing to the east, sits at the narrowest point of the Gaza Strip, well within range of Israeli tanks. It had been a focal point of prior clashes with Israel. During the first Palestinian uprising in 1987—the conflict that gave birth to Hamas—30 townspeople died. Clashes in 2009 and 2012 also took a toll.
Mr. Abu Thoraya, one of the Hamas fighters from Deir Al-Balah who died, was in some respects a typical young man in his 20s. He was unmarried, worked a clerical job and lived with his parents, whom he and his brother supported. He took long morning runs down the Gaza Strip toward Egypt.
He had a pious side which drew him to Hamas. He made connections to the group at the Abu Salim Mosque, an old stone prayer hall down the street from his home.
"We didn't share the same views," said his brother Mohammed, who is a member of Gaza's Dawah movement, which also is Islamist but doesn't have a militant wing.
Still, the family accepted and supported Mr. Abu Thoraya's decision to plumb the world of Hamas through Islamic study and religious training.
Mohammed Al Masri with a photo of his brother Abdullah, a 39-year-old Hamas fighter who was killed during a July 19 battle with Israel. Nick Casey/The Wall Street Journal
At some point, religious study transitioned into fighting. "You start as a fan of Hamas, then eventually, if they trust you, you join the armed movement," said his brother.
Three or four years ago, Mr. Abu Thoraya started disappearing for long periods, his brother said. Word started circulating at the mosque that he would one day be a "shahid," the Arabic word for martyr. "He liked the weapons," said his brother.
Abdullah Al Masri, who lived a short walk from Mr. Abu Thoraya's home, took a similar path. He was 39 when he was killed last month.
He worked as a police officer in the Hamas-run city bureaucracy and was known as the most devout of a strongly Muslim family. "We were almost brought up at the Abu Salim Mosque," said his brother, also named Mohammed. Following Friday prayers, Mr. Al Masri would spend the afternoon lecturing children on the virtues of Islam.
He joined Hamas more than a decade ago and told his family about the decision a few years ago, his brother said. "We were absolutely OK with this. There was an Israeli occupation that he needed to fight against," his brother said, citing the justification many Palestinians give for attacking Israel. The Israeli government considers any attacks for political purposes to be terrorism.
As this year began, there was talk that another war could be approaching. Hamas had lost a patron in Egypt when Islamist President Mohammed Morsi was deposed last summer. A new alliance between Israel and Egypt blockaded entrances into Gaza. In the spring, militant factions in Gaza, with consent from Hamas, began firing missiles into Israel.
In June, three Israeli teenagers were abducted in the West Bank, which Israel attributed to Hamas. Although Hamas denied that, Israel arrested the group's leadership there and later charged Hussam Al Qawasmi, who it said was the head of a low-level Hamas cell, with the crime. When the teenagers were found dead, Israel launched air attacks on Gaza and Hamas sent rockets into Israel.
On July 12, four days after the conflict began, Mr. Abu Thoraya called his brother. "I will be away for a while, and you need to take care of your father alone," his brother recalls being told. He took it as a signal that Mr. Abu Thoraya had been called up to fight, and he asked his parents to pray for their son.
The day before Mr. Abu Thoraya's death, the brothers saw each other one last time. "He said: 'I'm going somewhere," his brother recalled.
Something similar was going on in the home of Mr. Al Masri. He hadn't received his policeman's pay in four months, since Hamas ran out of money this year. His brother Mohammed called him to ask whether he could manage it. He said Mr. Al Masri asked him to pray for him, a sign he took to mean a fight was near.
Mr. Al Masri's mother, Latifa, was with him in the living room during the daytime Ramadan fast when they heard the sound of tank shelling outside Deir Al-Balah. "Are you afraid of it?" she said her son asked. "Because I'm not. What better thing than to be a martyr during Ramadan."
July 19 was a day of heavy fighting, the second day of Israel's ground invasion. Early on, a group of Hamas commandos entered Israel through a tunnel, killing two soldiers.
The event sparked reprisals from Israel, which later laid siege to a Gaza City neighborhood. Hamas fighters lured Israeli soldiers into corners of the Gaza Strip, intending to kill or kidnap them, according to both Israel and Hamas. It was the deadliest day in a decade of Israeli-Palestinian conflict, with more than 100 fatalities.
In Deir Al-Balah, Mr. Abu Thoraya's brother Mohammed was one of the first to hear of the attack on the brigade's fighters. He was told his brother had been injured by an Israeli fighter-jet attack in a field near Gaza's eastern flank.
He rushed to the town's hospital, where he found Mr. Abu Thoraya dead, with shrapnel wounds to his face and left leg. It was the first time he had seen his brother in his Hamas uniform: green trousers and a black T-shirt.
Mr. Al Masri's body was discovered by his own brother in the same hospital, missing most of the top of its head, his brother said.
A joint funeral was held immediately. A crowd of several thousand walked through the streets of Deir Al-Balah, holding the bodies aloft on stretchers, to the homes of each of the dead fighters.
The crowd then bore the bodies to Abu Salim Mosque. The afternoon prayer was recited, then a funeral prayer. Finally, the dead were taken to the cemetery for burial.
Several days later, Bilal Barouth, a grocer in Deir Al-Balah, heard a knock on his door. A member of the Al Masri family, he said, had come with the equivalent of $550—payment for Mr. Al Masri's bills for milk, diapers and other supplies bought on credit when he wasn't receiving his policeman's salary.
Mr. Abu Thoraya also had made provisions to settle his debts—a tradition among Hamas fighters. His brother found a note on his desk with a detailed list of clothes and phone chargers to be returned to friends. Five shekels—about $1.50—was owed to the grocery store. The rest of his money he left to his mother to make a pilgrimage to Mecca.
"I knew he was a member of Hamas, but I never saw any guns," his mother, Rayusa Abu Thoraya, said recently, while sitting on the floor of a relative's home with several grandchildren.
Mr. Al Masri's mother had a different response to her son's death. "God be praised," she said. "We knew he was part of the resistance and we knew the day would come that he would die."
Mr. Al Masri's brother, who works in the public-relations department at the local technical college, said he would never follow in his brother's footsteps to join Hamas's militant wing. But years of conversations with his brother over what the two saw as the futility of resolving the conflict by negotiations led him to respect his brother's decision, he said.
—Josh Mitnick contributed to this article.