The kidnapping of Marine Lt. Col. William Higgins has drawn our attention back to South Lebanon, where stability hangs by a slender thread. To a degree that is not widely appreciated, unarmed United Nations observers and peace-keeping forces play a profoundly important role in preventing yet another major eruption of violence in the Middle East. For more than three years, the U.N. forces have faced a steady and dangerous level of violence, much of it committed by Iranian-supported extremists who seek an end to the U.N. role.

A complex array of forces confront one another in South Lebanon. On one side are the Israelis and the Army of South Lebanon and on the other a mixture of Lebanese and Palestinian forces intent on undermining the security zone. In the middle is the United Nations Interim Force in Lebanon, which, with about 6,000 members, has been deployed in southern Lebanon since 1978. UNIFIL is supported by unarmed observers, such as Col. Higgins, who provide the world with unbiased reports on the situation and often play key roles in the resolution of disputes.

UNIFIL has provided a significant increment in security in the area. Even many of its Israeli critics have come to admit thatUNIFIL's role is a crucial one in stemming the level of violence. Israel's own unfortunate experience in Lebanon has no doubt helped many Israeli officials to readjust their standards in judging UNIFIL's effectiveness.

When the Israeli army completed its redeployment under pressure in June 1985 -- after being badly pummeled by a variety of Lebanese resistance forces -- it retained control of a "security zone" north of the Israeli border. The zone takes up nearly 9 percent of Lebanese territory and is a major magnet for armed attacks -- at the rate of 50 to 100 per month. Of course, Israel claims the zone is merely a security apparatus and denies that it has any designs on Lebanese territory, but viewed from the other direction the arrangement is a form of occupation.

The security zone in the south is the base for a proxy militia of about 1,500 Lebanese fighters under the command of Antoine Lahad, a retired general from the Lebanese army. Few informed observers believe that this militia, the Army of South Lebanon, is a particularly credible force. The ASL does not have a reputation for holding its ground when under attack, and without ample Israeli military support, including manpower, training and materiel, it would crumble. The force has persistently suffered from poor discipline and desertions.

For many residents of southern Lebanon, it is intuitively obvious that UNIFIL's presence is a key to preventing chaos. The force has created a zone of relative security and impressive economic vitality. Most important, UNIFIL acts as a buffer that reduces the intensity and frequency of violence. There is no question that UNIFIL impedes -- even if it is incapable of preventing -- Israeli military action.

The Shi'a Amal movement, in particular, has been very supportive of UNIFIL, which it sees as a surrogate for a legitimate, effective and functioning government. Amal is committed to restoring civility in the south, and UNIFIL is instrumental to that end. Although Amal has played a major role in resisting the Israeli presence in Lebanon, it has carefully circumscribed the realm of permissible resistance actions to the security zone itself, so as to preclude massive Israeli responses.

Unfortunately, the Iranian-supported Party of God (Hezbollah), which competes with Amal for the support of the Shi'a, is venomously critical of UNIFIL for allegedly serving Israel's, not Lebanon's, interests. From Hezbollah's perspective, were UNIFIL to be removed from the scene, the Israeli security zone would be even more exposed to attack by the Islamic Resistance Front. Extremists, inspired by this line of argument, have taken up arms against UNIFIL, and the level of anti-UNIFIL violence has reached troubling heights. No doubt the vehemence of anti-UNIFIL rhetoric also stems from an appreciation of the close working relationship that UNIFIL and Amal enjoy. In other words, it does not pass unnoticed that the immediate loser in the event ofUNIFIL's withdrawal would be Amal.

Amal is committed to preventing a reinstallation of the armed PLO presence that previously undermined civility and peace in the south. In fact, its siege of the Palestinian refugee camps in the Beirut area has arguably had as much to with thwarting PLO aims in the south as in the environs of Beirut. (The cultural epicenter of the Shi'a and the territorial base for the Amal movement are in south Lebanon, where about one-third of the Shi'a live.) It is no accident, therefore, that various Palestinian fighters and the Iranian-supported Hezbollah have sometimes found room for common cause.

Even when UNIFIL's vital role is understood, there is often a tendency to take the force for granted, to presume that it is a permanent fixture. It isn't. Sooner or later UNIFIL will be withdrawn, and there are troubling hints that its withdrawal could come as early as this year, especially if current trends are not markedly reversed. The departure of even one national contingent from UNIFIL would probably start an irreversible cascade of withdrawals, quickly leading to the dissolution of the force. When UNIFIL packs its bags, the ensuing power vacuum will prove irresistible to Israel, the PLO and the full range of Lebanese militias with interests in the area.

Barring a sudden burst of enlightened action, there is every reason for serious concern about the structure of security in southern Lebanon.

The writer, who served as a U.N. observer in South Lebanon, is associate professor of comparative politics at West Point Military Academy.