Each year in early winter, reserve Army Capt. Uzi Levin, in real life a Tel Aviv engineer, dons a military uniform for one month of active duty in this small border town, where he instructs visitors on the Israeli view of the political and military intricacies of southern Lebanon.

His students include American tourists bused on a three-hour daytrip from Jerusalem, representatives of American and European Jewish organizations and the occasional U.S. congressional delegation or journalist.

Business has picked up this year, following last summer's explosive cross-border exchanges between Israel and the Lebanon-based Palestine Liberation Organization. There is particular interest this week because U.S. negotiator Philip C. Habib is back in the region, shuttling among Beirut, Damascus, Jerusalem and Riyadh, trying to make the six-month-old cease-fire stick.

But Levin, echoing precisely what higher-ups say without their names attached, says he believes a long-term solution to the hostilities, or even continued maintenance of the cease-fire, may now be more difficult than ever.

Had Israel been given more time for its operations last July, just a matter of a few more days, Levin contends in his guided tour, Israeli bombers could have finished wiping out PLO strongholds in southern Lebanon to the point of no return.

Israel was dissuaded from doing so by U.S. pressure, through Habib. According to Levin and many other Israelis, that pressure was motivated by the United States' new friend in the region, Saudi Arabia, a supporter and funder, both directly and through Syria, of the PLO.

Since last summer, the United States has become even closer to the Saudis, approving the sale of AWACS radar planes and saying nice things about a Saudi Middle East peace plan, both over the strong Israeli objections.

The widespread feeling in Israel appears to be that the Saudi-U.S. friendship is based solely on American need for Saudi oil and is made possible through a misbegotten U.S. belief that the current Saudi regime is a stable and moderate one.

The Reagan administration, Israeli officials high and low say, will soon learn that Israel is the only stable and dependable U.S. ally in the Middle East. But in the meantime, they say, Israel is prepared to take care of its own problems here on the Lebanese border, Habib or no Habib, should it become necessary.

"Since July," Levin contends, the PLO has added "more than 200 artillery" pieces just north of the Litani River in Lebanon, "covering the whole northern part of Israel."

"We are complaining to the Americans all the time" about the PLO buildup to no apparent avail, he says.

Asked if Israel is contemplating a repeat of its fullscale 1978 invasion of southern Lebanon, perhaps this time all the way to the Zahrani River as rumored in Jerusalem, he shrugs.

Levin's tour takes us through the barbed-wire and electrified fence, guarded on both sides by Israeli soldiers, to a high hill inside the Lebanese border, just south of the town of Marjayoun.

The vista is breathtaking, with wide valleys and rolling hills and ridges covered by scrub and boulder. The terrain, not surprisingly, matches that of scores of American-made films of biblical history that took place not far from here around the Sea of Galilee.

So broad is the view and so compact and complicated are the various allegiances and frontiers in this region, that nearly all of what appears on a large map Levin spreads across the trunk of a car can be seen in reality with the naked eye.

Facing north, to our far left along high ridges are the strongholds of breakaway Lebanese Army Maj. Saad Haddad, on whose territory we also are standing. Since 1978, when Israel withdrew its forces to behind the border under a U.N. agreement, Haddad has controlled what he calls Free Lebanon, a strip three to 14 miles wide on the Lebanese side.

Haddad's enemies are the PLO, which he calls occupying strangers in his country and, indirectly, the Lebanese government, which he considers illegitimate and impotent. His friend is Israel, which supplies his allegedly several thousand strong Army and militia of Lebanese Moslems and Christians in exchange for the buffer he puts between the PLO and the border.

Although the United Nations and much of the rest of the world insist that "Free Lebanon" is an Israeli ploy to violate the 1978 agreement, Israel and Haddad insist that Haddad is a free agent and a Lebanese patriot.

Up here on the border, however, those are the kind of niceties reserved for diplomats and speeches. Although the Israeli military denies charges that its troops are set up inside Lebanon, there is no doubt Israel has a degree of free access to Haddad's territory.

"I'm here," acknowledges Levin with a smile as we stand atop the hill. "Did we get visas from Beirut?" In any case, he says, "Everybody violates the agreement all the time."

To make his point, he gestures into the distance, slightly to the right of Haddad's emplacements, to another hill in the northwest. At its summit is an imposing Crusader-era stronghold, Beaufort Castle. It is a well-fortified PLO outpost, Levin says, whose guns are trained across Haddad's strip on settlements in northern Israel.

Under the U.N. agreement, the hill and miles behind it are part of the territory supposedly controlled by the 6,000-man U.N. peacekeeping force, which is charged with keeping the area clean of belligerents. According to Israel and Haddad, as well as journalists who have visited there and the PLO themselves, Beaufort Castle is the most blatant example of the PLO presence througout the U.N. zone.

Beyond the valley to our right, where Lebanese farmers plow small fields, appearing stubbornly oblivious to the danger, is snow-covered Mt. Hermon, marking the northern edge of the Israeli-occupied Golan Heights. The Golan is separated from Syria, which lost it to Israel in the 1973 war, by another U.N. buffer zone.

Along the Golan, Israel has a similar lack of confidence in the United Nations' ability to keep the peace. Dotted along that mined and wired border are a string of bunkers where Israeli soldiers stare through binoculars across the U.N. zone at their Syrian counterparts staring at them.

The Golan border is quiet now. The new trouble with Syria is far to the north, where Syrian antiaircraft missiles installed last May provoked a related crisis with Israel that Habib also is trying to iron out.

Uzi Levin spent his reserve duty here last year and the year before and he will most likely spend it here next year, too. When he is back in Tel Aviv -- he worries that his business is stagnating in his 28-day absence -- other parttime officers take his place.

In Metullah, a busload of Americans has pulled up among the jeeps and olive-painted trucks on the main street, brought from Jerusalem to see the frontier area and the soldiers and to let Israel tell them what is going on here.

They are immediately disappointed, as Levin tells their leader he cannot take them "inside" today, across the border, and that their anticipated meeting with Haddad has been canceled. Minutes before, Haddad had hopped into a jeep across from their bus with a group of Israeli officers and roared off toward "Free Lebanon."