His role underlines the family core of the Assad regime, though he is a stark contrast to his brothers. His eldest brother, Basil, was the family prince, publicly groomed by their father, Hafez, to succeed him as president — until Basil died in a 1994 car crash. That vaulted Bashar, then an eye doctor in London with no military or political experience, into the role of heir, rising to the presidency after his father's death in 2000. The two brothers — the "martyr" and the president — often appear together in posters.
The 45-year-old Maher Assad, however, has resolutely stayed out of the limelight. Friends, military colleagues and even his enemies describe him as a strict military man to the core.
The 15,000 soldiers in the 4th Armored Division that he leads are largely members of the Assad family's minority Alawite sect — who see the civil war as a battle for their very survival — and represent the best paid, armed and trained units of the Syrian military. In the past year, his troops have launched repeated offensives against rebels firmly entrenched on Damascus' outskirts, bombarding and raiding the impoverished suburbs they hold.
Maher Assad is also believed to have led a bloody crackdown on dissent since the uprising began in March 2011 with largely peaceful protests against Assad's rule. In April, the Syrian rights group Violations Documentation Center reported interviews with several former detainees who described being crammed in crowded cells and undergoing beatings by guards in secret prisons on the 4th Division's bases around Damascus, where hundreds of suspected regime opponents have been held.
"He is known to be a merciless butcher," said Mohammed al-Tayeb, an opposition activist speaking by Skype from the rebel-held Damascus suburb of Douma, among the areas pounded by the 4th Division's assaults.
Within President Bashar Assad's circle of trust, Maher Assad has advocated an uncompromising response throughout the uprising.
"From the beginning, Maher was convinced that the uprising must be put down before any talks take place," said Fawaz A. Gerges, director of the Middle East Center at the London School of Economics. "The life of the regime depends on Maher's ability to prevent the rebels from infiltrating Damascus and toppling his brother's government. ... If Damascus falls, the regime goes,"
He also played a role in reshaping the Syrian military as the conflict dragged on. Once plagued by defections as rebels gained territory, the military has regained the upper hand this year with a series of powerful offensives, battling rebels to a standstill in cities and taking back some towns.
"The Syrian military has changed from a rusty institution, filled with passive and tired conscripts into an urban warfare fighting machine, filled with skilled and battle-hardened fighters," said Gerges.
Maher Assad's importance has only grown. His brother-in-law, Assef Shawkat, who was deputy defense minister and a key figure in the intelligence apparatus, was killed along with the defense minister in a June 2012 bombing. Shawkat's wife, the Assads' older sister, Bushra, herself a major adviser to the Syrian president, is believed to have since left the country for the Gulf. Several of Bashar Assad's cousins hold significant security posts, but Maher Assad is by far the most prominent relative.
Following the Aug. 21 alleged chemical attack near Damascus that killed hundreds, opposition activists charged that the rockets carrying the chemical agents were fired by the 4th Division's 155th Brigade, which commands large missile sites on the mountains overlooking the capital. However, the opposition could not produce proof.
The United States blames the military for the attack but has not specified which units — though Maher Assad's are the ones that operate in the capital. The Syrian government has denied its troops carried out the attack, accusing foreign Islamic militants among the rebels.
Maher Assad's relationship with his 48-year-old brother in some ways mirrors that of his father's to his own younger brother, Rifaat Assad, who commanded an elite military unit and was seen as the regime enforcer in the first decade after the Assads came to power in a 1970 coup. But Rifaat Assad fell out with Hafez Assad after he made his own bid for power in the mid-1980s, and he has lived in exile in Europe since.
Maher Assad, a brigadier general, has shown no similar thirst for the presidency, and there's been no public sign of friction with his brother.
Last year, when rebels were striking directly into the heart of Damascus, a few startling chants calling for Maher Assad to rule could be heard at pro-government protests: "Bashar to the clinic, Maher to the command." They underlined a perception that the military younger brother was more fit for power than his former doctor brother. But the chants were scattered, soon ended and never grew to a larger campaign.
From behind the heavy secrecy, numerous reports of Maher Assad's ruthlessness and temper circulate — though few have been confirmed.
Opposition figures and activists say Maher Assad was behind a July 2008 crackdown on a riot by mainly Islamist prisoners at the Saidnaya prison near Damascus, in which at least 17 prisoners were killed, according to Amnesty International. Others put the toll at several dozen.
An online video of the scene shows a man activists claim is Maher Assad taking cell-phone pictures of the mangled bodies and severed limbs of the dead. The man resembles Maher Assad, though it has not been confirmed to be him.
Hisham Jaber, a retired Lebanese army general who has studied the Syrian army and is in touch with officers from the 4th Division, said Maher Assad is known as a "brave ... and in some respects aggressive man, who has a lot of military experience." He is respected by his troops but also feared for his strictness, Jaber said.
Like his brother, Maher Assad is married to a member of Syria's Sunni Muslim majority. He lives with his wife, Manal Jadaan, and their three children — two girls and an 18-month-old boy — in a villa near the presidential palace in Damascus. Maher Assad is a passionate equestrian and owns a ranch and horses in the Yaafour area, near Damascus, according to two family friends, who spoke to The Associated Press on condition of anonymity to give details of his private life.
He also collects autographed jerseys from athletes and old motorcycles, they said.
Qassem Saadeddine, a former Syrian army colonel and spokesman for the rebels' Supreme Military Council, counters that Maher Assad ensures loyalty among those close to him with largesse. "He gives them money, cars, houses and all means of entertainment."
"He controls the country and its resources, that's what he does."