Approximately 15,000 armed Palestinians. That's the size of the military force the Israel Defense Forces will face if a major operation in the Gaza Strip goes forward. These militants, from various Hamas factions, will presumably be aided by a few thousand militants from other Palestinian groups.

For two years Hamas, with Iranian assistance, has been working hard on developing its military power, using Hezbollah as a model.

Gaza Palestinians are preparing to step up their offensive, with rockets and mortar shells directed at Israel's civilian population in the south, as well as their defense, digging in to retard the IDF's progress and cause heavy Israeli casualties.

Nevertheless, military experts in Israel and the West believe the IDF is capable of retaking Gaza. Israeli reservations about a broad military operation, therefore, are mainly linked to the question of what happens afterward, when the IDF controls a large area that it doesn't want and is in constant friction with terrorists and the civilian population.

The main components of the "Hamas Army":

? Order of battle: Hamas is transitioning from a terror group to a paramilitary guerrilla organization. The transition includes improvements to the command and control structure, the acquisition of better weapons and the creation of a training program.

The core of Hamas' "army" is its military wing, Iz al-Din al-Qassam, which the organization sees as its best trained and most disciplined force. It was deployed against Fatah in June 2007 and it will bear the brunt of any engagement with the IDF. Iz al-Din does not generally perform unpopular policing operations (such as the daily suppression of Fatah), instead focusing on preparing for battle with Israel.

The estimated size of the force is about 1,000, divided into sectors and from brigades down to companies.

? Training: Palestinian sources say Iz al-Din troops undergo rigorous military training as well as participating in ideological classes held in mosques. Hamas forces do six months of basic training that includes live-fire exercises in which they learn to fire rockets, antitank missiles and mortar shells.

They undergo urban warfare training, including exercises simulating an assault on a settlement complete with covering machine-gun fire and antitank fire prior to the assault. Some of the instructors were trained in Iran and Lebanon. In recent years dozens of Gazans have traveled to training camps run by terror organizations and Iran's Revolutionary Guards.

IDF soldiers who have fought Hamas cells in the Gaza Strip in the past two years report an impressive improvement in their discipline and in their equipment.

? Other factions: Hamas and smaller organizations, starting with Islamic Jihad, can be expected to cooperate in the event of an engagement with the IDF. Two Popular Resistance Committee factions maintain close contact with Hamas and are likely to subordinate themselves to the organization in a war with Israel.

Three military groupings identified with global jihad (that is, Al-Qaida and its offshoots), on the other hand, will not accept Hamas authority and will continue to operate independently.

? Rocket attacks: The rockets and mortar shells were initially developed as a way of bypassing the border fence, which prevented militants from entering Israel to carry out attacks. Shin Bet security service head, Yuval Diskin, told the cabinet this week that Hamas already has rockets with a range of 40 kilometers, that are capable of reaching Ashdod and the outskirts of Be'er Sheva.

These are advanced Katyusha that were smuggled into Gaza in pieces through the tunnels and assembled in Gaza. These rockets have not yet been fired at Israel but Hamas and Islamic Jihad already have dozens of 122 mm. Grad rockets with a range of about 20 km. Gaza militants recently began using another Iranian-supplied weapon, 120 mm. mortar shells with an 8-kilometer range.

Hamas has also made significant gains in manufacturing its own rockets. It has learned to create Ammonium Perchlorate Compound, an advanced rocket propellant that in addition to extending the Qassam's range beyond 20 kilometers also - and more importantly - increases the rocket's shelf life.

That means the organization can, for the first time, maintain a supply of rockets for months at a time. Analysts believe Hamas currently has over 1,000 rockets. Islamic Jihad maintains its own production and storage facilities, but both rely on Iranian experts for training. Sources in Gaza say that Hamas' "military industry" is working overtime to manufacture rockets, and that the organization can easily fire 80 rockets a day, as it did on Wednesday.

? Defense: Hamas' defensive strategy includes an extensive underground network of bunkers, tunnels and booby-trapped structures. The Palestinians have proved their explosives capabilities, having destroyed three Israeli tanks and two armored personnel carriers using high-grade explosives.

Antitank missiles are an important component of Hamas' defensive strategy, which takes on board the lessons learned by Hezbollah in the Second Lebanon War. Hamas has acquired antitank missiles from the Eastern bloc, although the exact models and capabilities are not known.

Militants can be expected to employ antitank missiles against Israel Air Force helicopters in the event of a confrontation, in the effort to delay and obstruct the entry of the IDF.

? Offensive plans: Hamas' main weapon is its ability to launch dozens of rockets a day at Israel. In the event of an escalation the organization can be expected to try to prove that it can hit more distant targets, such as Be'er Sheva. Ashkelon is liable to suffer massive rockets attacks. In addition, Hamas is likely to target one kibbutz or moshav near the border in an attempt to cause large numbers of residents to leave and weaken Israeli morale. Israel must also be prepared for a surprise from Hamas, a la Hezbollah, ranging from additional tunnels to facilitate abductions to attacks on boats or aircraft and up to attacks on strategic targets in the south.