When Israel pulled its forces out of its occupation zone in southern Lebanon in May 2000, fighters of Lebanon's Hizballah organization swiftly took up position along the Blue line, the United Nations-delineated boundary separating Lebanese territory from Israel and Israeli-occupied Syria.

The blue line is the locus of direct military confrontations between Hizballah's experienced, battle-hardened guerrilla fighters and the Israel Defense Forces (IDF). Syria sanctioned Hizballah's deployment along the blue line as a means of continuing to needle Israel, despite the IDF's UN-approved withdrawal from south Lebanon. The Golan Heights remains in Israeli hands, and for Damascus, Hizballah remains one of the few, if not the only, potent bargaining chips with which to pressure the Jewish state into returning the strategic plateau.

Hizballah's presence along the blue line is comprehensive although largely inconspicuous. Fighters, usually in civilian clothes and equipped with walkie-talkies and binoculars, man some 25 small observation posts along key points of the 70-mile blue line from where they monitor Israeli border outposts and troop movements. Other Hizballah members, armed and in full combat gear, patrol the remoter stretches of the border.

Since 2000, Hizballah has built up a considerable arsenal of conventional weapons and ammunition. Israel says Hizballah also possesses a substantial number of long-range rockets, capable of striking targets deep inside Israel. There is no independent confirmation that these rockets exist in Lebanon, although given Hizballah's strategic priorities it is quite likely that it has long-range artillery capabilities.

Hizballah justifies its military operations along the blue line as resistance to Israeli occupation or defending Lebanese sovereignty from Israeli aggression. The distinction is important for Hizballah as it shifts the onus of responsibility for any aggression along the blue line onto Israel.

The main area of direct military confrontation between Hizballah and the IDF is in the Shebaa Farms area, a 15-square-mile mountainside running along Lebanon's southeast border with the Golan Heights. The Shebaa Farms are claimed by Lebanon, although the UN decreed in 2000 that the area belongs to Syria.

Hizballah periodically attacks mountaintop IDF outposts with anti-tank missiles, Katyusha rockets and mortar rounds.

Israel's persistent penetrations of Lebanese airspace with aircraft and reconnaissance drones are another source of confrontation. In response to the overflights, Hizballah anti-aircraft gunners occasionally fire 57mm rounds across the border. The rounds explode in the air thousands of feet above Israeli towns, spattering whatever lies below with light shrapnel.

Hizballah also uses the blue line as a means of retaliating for Israeli actions beyond south Lebanon, such as assassinations of party officials and major developments in the Israeli-Palestinian conflict.

Linking its actions to broader developments in the Arab-Israel conflict serves to bolster Hizballah's pan-Arab resistance exemplar while warning Israel that its actions cannot be isolated from the region as a whole.

In May, Hizballah staged its boldest operation yet against IDF troops, luring a patrol of elite Israeli Egoz commandos (a unit established in 1995 specifically to fight Hizballah guerrillas in south Lebanon) across the blue line in the Shebaa Farms before attacking them with roadside bombs and anti-tank missiles. Although it was a well-planned trap designed to inflict casualties, Hizballah's statement on the incident portrayed it as a defensive measure against an IDF incursion onto Lebanese soil.

The blue line remains a source of tension and is susceptible to periodic escalations in the simmering conflict between Hizballah and the IDF. But there are constraining factors on either side that help maintain the status quo.

For Israel, a major assault against Hizballah (a dream of some Israeli military commanders still smarting from the ignominy of the May 2000 withdrawal from Lebanon) cannot guarantee the destruction of the Lebanese group, especially if it continues to receive the political backing of Damascus. Militarily, Hizballah's status as a guerrilla organization makes it a difficult target for conventional forces. Furthermore, Hizballah has created a "balance of terror" with its suspected long-range rocket arsenal that can reach major urban centers in Israel, a grim reality that IDF commanders have to take into account.

But Hizballah too has constraints, chief of which is not to aggravate the situation along the blue line to the extent that Syria is dragged into a war with Israel. Hizballah's freedom of action is also limited by domestic considerations. The main reason it describes its activities along the blue line as championing Lebanese sovereignty is to appease its war-weary Shiite constituency and those Lebanese critics who remain suspicious of the organization's intentions.

Although both sides occasionally tinker with the rules defining the conflict, neither side appears to be willing to tear them up at the present time. As long as that remains the case, Hizballah's actions along the blue line will continue to be a source of irritation to Israel rather than a genuine threat.