Israel has Lt.-Gen. Gabi Ashkenazi as its chief of staff. Hamas has Ahmed Ja'abri. Ja'abri is in his late 40s and has been in Israel's sights for a number of years. In 2004, Israel Air Force jets fired several missiles at his home in the Sajiya neighborhood of Gaza City. Ja'abri escaped the assassination attempt with moderate wounds. Five others were killed. Since then, he has slowly climbed the Hamas ranks and today is believed to be the group's "chief of staff," replacing arch-terrorist Muhammad Deif, who was seriously wounded by an Israeli air strike in July 2006 and whose role in the organization today is unclear. Ja'abri is credited with the current Hamas build-up and is believed to be far more extreme than its political echelon. Today, the group has five brigades corresponding to five sections of the Gaza Strip - North, Center, Gaza City, and two brigades in the South. Each brigade has a commander and several battalions. Alongside the battalions there are special forces - units with expertise in rocket fire, mortar attacks, roadside bombs and commando operations. In Gaza, the IDF would face an army of close to 20,000 armed men, among them at least 15,000 Hamas operatives. The rest are from Islamic Jihad, the Popular Front for the Liberation of Palestine and the Popular Resistance Committees. Ja'abri is the commander of Gaza City. Ahmed Andour is in charge in the North. Iman Nufal, the commander of central Gaza, is in Egyptian custody after he was arrested last year when he entered Sinai. The two brigades in the South - one in Khan Yunis and one in Rafah, are led by Mahmoud Sanour and Ra'ad Alatour. Andour is believed to be Ja'abri's right-hand man. He was reportedly imprisoned by the Palestinian Authority for five years in the mid 1990s. Together with Ja'abri, Andour was behind the attacks against Fatah militiamen in the summer of 2007 that led to Hamas's takeover of the Gaza Strip. Ja'abri and Andour are believed to have masterminded the June 2006 kidnapping of Gilad Schalit. Nufal was one of the organizers of the January 2008 breaching of the border wall between Gaza and Sinai, which is why Egypt continues to keep him in prison. In July, Hamas revealed that a group of Palestinians had been arrested for allegedly plotting to kidnap Rafah commander Alatour. Hamas Interior Minister Said Siam revealed in an interview to Al-Hayat that the plot was exposed and thwarted by Hamas security forces. Sanour, the commander in Khan Yunis, is believed by Israel to have been responsible for hundreds of terror attacks in recent years, most prominently the March 2002 infiltration of a pre-military academy in the Gush Katif settlement of Atzmona in which five Israeli students were killed. IDF officers like to say that Israel can, without a doubt, conquer the entire Gaza Strip within days. The difficult part is holding on to the conquered territory against Hamas's guerrilla style of warfare. Both Israel and Hamas have used the past six months of the cease-fire to build up their military capabilities and for extensive training. The Golani Infantry Brigade, which is supposed to deploy on the Gaza front in the coming weeks, as an example, just completed four months of intensive training in the South and the North ahead of potential conflicts with Hamas, Syria and Hizbullah. Conquering the Gaza Strip would involve several brigades and all branches of the IDF - the air force, Artillery Corps, Armored Corps and, of course, infantry. Since the unilateral disengagement from the Gaza Strip in 2005, Hamas has been involved in one of the most intensive military buildups - for a terrorist group - in modern history. It, everyone knows, is no longer a small terrorist group just capable of building explosive belts for suicide bombings. While Hamas used the past six months of a "cease-fire" to train its forces, it also took advantage of the suspension in IDF operations to fortify its military posts in the Gaza Strip. According to one high-ranking security official, Hamas has dug dozens of kilometers of tunnel systems throughout Gaza that will be used by fighters to move from one place to another undetected. "Just like the Vietcong," the official noted. Hamas has also dug foxholes throughout the Strip for anti-tank missile units as well as for massive bombs that have been placed on the main roads into Gaza. "Hamas has learned a lot from Hizbullah and has adopted many of the Lebanese group's tactics that were used successfully against the IDF in the Second Lebanon War," one official said. An indication of Hamas's intensive training regimen has been the series of "work accidents" in Gaza throughout the cease-fire. On July 8, for example, two Hamas operatives were killed by an explosion in a training camp in Khan Yunis. Another explosion took place on July 30. also in the Khan Yunis area. Six operatives were wounded. The IDF believes that in both cases, the terrorists were trying to make large bombs or practicing with new explosives and rockets.