Earlier he allied with Fatah. In 1982, he was imprisoned for 13 years. He joined the Muslim Brotherhood before Hamas was established.
He met future Qassam Brigades leader Salah Shehadeh and other Hamas figures. He became politically and militarily active. After release from prison in 1995, he ran a Hamas-affilated organization. It worked for current and former prisoners. In 1997, he worked with the Hamas-connected Islamic Redemption party.
In 2002, an Israeli air strike killed Shehadeh and seriously injured his successor, Mohamed Deif. In August 2004, Jabari gained Qassam Brigades operational control. He built it into a 10,000-strong force.
He helped Hamas maintain control of Gaza after Abbas established an illegitimate West Bank coup d'etat government. He's known as Qassam Brigades commander.
He was actually second in command to the figure who hasn't been seen since the early 1990s. Called "the general," he survived several earlier assassination attempts.
In 2004, Israeli helicopters targeted his Shujaiyya home. He was slightly wounded. His oldest son, Muhammad, his brother, and three other relatives were killed at the time.
He gained prominence for his role in capturing Gilat Shalit and negotiating his release in exchange for 1,027 Palestinian prisoners. After Cast Lead, he gained greater political influence.
Haaretz writer Jack Khoury asked "Who was Ahmed Jabari? He was well known and respected in Gaza. He didn't gain prominence until negotiations for Shalit's release. Many considered him the most important military and political Hamas leader.
"A former inmate who shared his prison cell told Haaretz that Jabari was a dominant figure in the prison."
"We would sit and drink tea. It was a different period, before the first intifada, and he wasn't considered a leader at that time and didn't think he would reach such a senior position."
In Gaza, he was active in Hamas' charitable work. He was in charge of coordinating fundraising and military activities.
He became influential in establishing ties with senior Hamas figures abroad. In 1998, the PA arrested him. He was released when the second Intifada erupted in September 2000.
Israeli peace activist Gershon Baskin said Israel erred in killing him. Lives on both sides will be lost.
Hours before he was assassinated, he got a truce agreement draft. It included ways for establishing ceasefire in case of future flare-ups.
Israel knew he was working with Egypt to establish permanent truce. Nonetheless, the killed him.