On dirt roads cut through rugged hills, overweight armored vehicles taxi Israeli soldiers from base to sandbagged base. Bulked up with extra armor plates against missiles and roadside bombs, the half-tracks strain to pull their loads and look like metaphors for an army bogged down in guerrilla war: muscle-bound, defensive, far less agile than its tormentors in the orchards nearby.

"They know the terrain better," said a sergeant whose Golani brigade has lost seven soldiers this year to the Shiite Muslim militia of Hezbollah, or Party of God. "It's their turf, so they have an advantage."

Israel's long frustration here, in the southern Lebanon "security zone" declared in 1985, accounts in large measure for the outgoing howitzer fire that has half-drowned conversations for the last 10 days at Israel's command post at Tel Nahas, north of Metulla, Israel. Stalemated in a war of attrition on the ground, Israel took to the air on April 11, with bombers and long-range artillery.

Prime Minister Shimon Peres cast Operation Grapes of Wrath as an answer to Katyusha rockets fired by Hezbollah at Israel's northern Galilee. But the story of how the offensive began appears to have as much to do with events inside the 328 square miles of Lebanon that Israel rules with a proxy militia called the South Lebanese Army (SLA).

There are many explanations of why the violence burst out of its confinement in the security zone: Hezbollah's growing boldness, Iran's strategic aims, Syria's ire at a diplomatic freeze, Israel's election-year ambitions and fears. Underlying all of them was the instability of the security zone itself, where Israel was in danger of losing its grip.

In a rooftop briefing at the Tel Nahas command post the other day, Brig. Gen. Giora Inbar, Israel's commander in the zone, did not even mention attacks on northern Israel when asked to explain the outbreak of the war.

"We started this operation after the situation in southern Lebanon became intolerable," he said. "South Lebanese Army soldiers, Israel Defense Force soldiers and civilian citizens here all over the security zone couldn't go on living under the threat of Hezbollah shelling, bombing and blows to their villages."

When Israel invaded Lebanon in 1982, hoping to strike a death blow to the Palestine Liberation Organization, there was no enemy called Hezbollah. The PLO's state-within-a-state had few friends among Lebanon's Shiite Muslims, and the Shiites largely sat out the war.

But by the time Israel pulled out its forces three years later, Lebanon's Shiite plurality had grown hostile. To keep them at a distance, Israel set up a security zone amounting to about 10 percent of Lebanon.

Today that border strip is ruled by Antoine Lahad, 67, a pompadoured Christian claiming the rank of general who tends to make public appearances in double-breasted European suits. Lahad's 2,400-strong South Lebanese Army, equipped and paid by Israel and supported by 1,000 Israeli troops, is the sole armed force in the security zone save for blue-helmeted United Nations troops. The SLA and its Christian-dominated institutions conscript soldiers, collect taxes, supply utilities and run hospitals and south Lebanon's only jail.

Hezbollah, founded as a terrorist group, has entered the mainstream of Lebanese politics. Today it holds seats in parliament and runs a network of hospitals and schools. It has its own television and radio stations, whose broadcast antennae have been among Israel's targets in recent days. And it has a military wing that long since took the lead in trying to expel Israel from the security zone.

Israel has said for many years that it harbors no territorial claims on Lebanon and will gladly withdraw from the occupied strip once assured that the border region will not be used for infiltration and rocket attacks. Many Lebanese, even those unsympathetic to Hezbollah's call for strict Islamic rule, regard armed resistance as justified until Israel departs.

"If he occupies part of our country, it is not self-defense," Prime Minister Rafiq Hariri told CNN on Thursday. "So {Israel's} attack on Hezbollah and Hezbollah's attack is in fact a fight between a resistance and the occupation."

"Why can't the Americans understand?" Walid Jumblatt, a cabinet minister and political chieftain of Lebanon's Druze community, asked in an interview. "It's like Vietnam."

One point of resemblance is the ferocity of the guerrilla war. Neither side takes prisoners, as Inbar acknowledged in another interview nearly a year ago.

"When there are short-range clashes, either we kill them or they run away," Inbar said.

Hezbollah sets ambushes and bombards civilian targets, both in northern Israel and Israeli-controlled Lebanese towns such as Marjuyun. Israel -- although officials say it does not aim to do so -- is so indiscriminate in its use of firepower that it has killed considerably more civilians than Hezbollah. Each side describes its use of weapons as retaliation.

Israel's preference for long-range combat, and Hezbollah's habit of fighting from the close environs of civilians, mean that Israeli and SLA tank and artillery fire often strike inside Lebanese villages, and noncombatants suffer.

There are no reliable statistics, but U.N. peacekeepers and the American-based Human Rights Watch/Middle East have documented numerous examples of "retaliatory shelling" by Israel that killed and maimed Lebanese children and elderly civilians.

Lahad, the SLA commander, explained these incidents last year by saying that sometimes "a shell goes astray." Uri Lubrani, the former Mossad deputy chief who has run Israel's Lebanon policy for many years, had another explanation: "This is not a tennis match."

Israel sometimes apologizes in such cases, and occasionally it announces that an officer has been disciplined for careless fire. According to U.N. officials, who log every exchange, Israel also let several cases pass in which Hezbollah rocketed northern Israel in what it said was retaliation for Lebanese civilian deaths.

In July 1993, after the last major Israeli offensive in Lebanon, the United States brokered "understandings" on the conduct of the guerrilla war. Negotiated by telephone by Secretary of State Warren Christopher, in successive conversations with Syrian President Hafez Assad and Yitzhak Rabin, then the Israeli prime minister, the understandings remained unwritten and have never been fully described in public. Their gist was that neither side would target civilians, but Hezbollah interpreted them to mean it could rocket Israel if Israeli fire harmed civilians in Lebanon.

Hezbollah, since then, has grown far more proficient and aggressive as a military force. Iran acknowledges support for the group, which a senior Israeli intelligence officer estimated at $100 million a year.

Hezbollah fighters have an arsenal appropriate for their guerrilla war. Most of their weapons can be carried by hand, and others need only a pickup truck to move. They use night-vision equipment, Sagger antitank missiles, heavy machine guns, 82mm and 120mm mortars, and several variants of the Katyusha rocket, including 122mm models.

During the 1990s, Hezbollah has launched more attacks in the security zone every year, and every year killed more Israeli and SLA troops. In 1994, the last year for which full data were available, Hezbollah killed 21 Israelis and 43 of Lahad's soldiers, compared with 12 and 13 two years before. In 1995, 23 more Israeli soldiers died, and another seven in the first quarter of this year.

"Now it's scary," said Eyal Hasid, 18, an Israeli enlisted man on his way into Lebanon this week. "The Hezbollah is getting better and better."

Israel's technological advantages, while considerable, have not sufficed to silence Hezbollah's small arms, rockets and bombs. Israeli electronic-warfare aircraft used broad-band transmissions to detonate some of Hezbollah's radio-triggered bombs for a while, but then Hezbollah learned to keep the trigger disarmed until an ambush was imminent. Counter-battery radar allows the Israeli army to shoot at Katyusha launch points within seconds of an attack, but Hezbollah now uses homemade timers -- typically involving a wristwatch and a motorcycle battery -- so that, as U.N. spokesman Timor Goksel said, the fighter is "home watching television" when the rocket is launched.

Moreover, there is strong evidence that Hezbollah intelligence has penetrated Lahad's force. The guerrillas often seem to know where and when Israeli and SLA patrols will come, and they especially like to strike new units as they rotate into the zone.

Along with growing casualties, a sense of imminent abandonment caused SLA morale to plunge. Israeli-Syrian negotiations were moving throughout 1995 and early this year toward a land-for-peace deal on the Golan Heights that was generally expected here to include an Israeli withdrawal from the security zone. "As far as the Israelis are concerned," Lahad said, "they are much more interested in peace with Syria than in the future of Lebanon."

While their Washington talks continued, Assad and Peres both had reason to keep the intensity of the fighting below a boil. But when Peres suspended the talks last month, after a series of terror bombings by groups with leaders in Damascus, Assad lost his incentive to restrain Hezbollah. And Peres, when he considered a new offensive, did not have talks with Syria to protect.

It was in this context that the war of attrition began to heat up in mid-March. During a U.S.-led anti-terrorism conference held March 13 in the Egyptian resort town of Sharm el Sheikh -- a milestone of Israeli-Arab cooperation -- Hezbollah launched its largest coordinated offensive in years.

Two weeks later, on March 30, Israeli gunners killed two civilians in Yatta. Peres went on television to apologize, trying to tamp the crisis down. But Hezbollah fired a Katyusha barrage, and thousands of tourists canceled plans to spend the Passover holiday in the Galilee. Said Hasan Nasrallah, the Hezbollah leader, broadcast a warning that Israelis should stay in their bomb shelters because he intended to hit them again.

The same month saw a failed car bombing with 880 pounds of TNT, then a Hezbollah hang glider who aimed for northern Israel but got tangled on a power line and blew up, and a successful suicide attack that killed an Israeli officer in the zone.

Through diplomatic contacts in Tel Aviv and Washington, Peres asked the Clinton administration to intercede with Syria to stop what he saw as a rapidly escalating crisis. Assad, according to officials from both countries, delivered nothing.

Neither Israel nor the United States, according to officials from both countries, wanted an explicit agreement in advance about Operation Grapes of Wrath. Instead they had an unwritten understanding: When Peres stopped asking Christopher to appeal to Syria for calm, it would mean Israel was preparing to strike.

"We did not want to be criticizing Israel for responding to aggression funded and directed by Tehran with the assent of Syria," a U.S. official said. "We were not going to use the word restraint' in our comments, and we were going to give the Israelis some running room."

Israel's security establishment had been itching to step up the fight for months. Lubrani, a craggy former intelligence officer who chain-smokes Marlboros and drinks a bottomless cup of Turkish coffee, was advising Peres to "give them a walloping and say to hell with it."

Peres is running a close race for reelection on May 29, and the swing vote was thought to be people torn between the hope for peace with the Arab world and fear about Israel's security in the new Middle East. Peres, whose peace credentials were not in doubt, was running on this slogan: "A strong Israel with Peres."

Many analysts agree that while Peres had something to gain from Operation Grapes of Wrath, he had more to lose if he failed to respond to Hezbollah's escalation.

"Even though I'm no great sympathizer of Shimon Peres, I don't believe he did this for the elections," said Tel Aviv University strategist Dore Gold. "I think he wanted to hold the lid on the pot until after the elections, for the simple reason that it's part of his election strategy to demonstrate his good ties to the Arab world."

The trigger came April 8, when a 16-year-old Lebanese boy was killed by a mysterious explosion in Barasheet. Israel described the explosive as an old mine or shell; Hezbollah accused Israel of planting a bomb. On April 9, the guerrillas loosed the deadliest Katyusha barrage into northern Israel in more than two years, inflicting 34 casualties. Israeli television and still cameras recorded the scene when the deputy mayor's wife in Kiryat Shemona was pulled with critical injuries from her burning car.

Israel's diplomatic channels to Washington fell silent. The Clinton administration did nothing to intervene. Two days later, after quietly evacuating children from Israel's northernmost towns, Peres launched Operation Grapes of Wrath. Gellman reported from the security zone, Israel's northern Galilee, Jerusalem and Tel Aviv. Lancaster reported from Beirut and southern Lebanon. Staff writer Thomas Lippman in Washington also contributed to this report. CROSS-BORDER DUELS Hezbollah rocket attacks April 9 Islamic militant guerrillas of Hezbollah begin launching Katyusha rockets from Lebanon into northern Israel. Rocket attacks are concentrated on Kiryat Shemona. Katyusha BM-21 Rocket Launcher

* Originating in former Soviet Union, Katyusha rockets entered service in the early 1960s. They are fired from a truck chassis. The missile's range is 12.7 miles. It can carry 42.8 pounds of explosives. SOURCE: Modern Land Combat, Jane's Armour and Artillery Israeli military response April 11 Attacks draw massive Israeli retaliation: Israeli helicopters and jets rocket Beirut's southern suburbs for the first time in 14 years. Towns and villages in southern Lebanon, including Nabatiyah and Tyre, also are hit. Hezbollah base in Baalbek, northern Bekaa Valley, is attacked. Civilians in southern Lebanon, heeding Israeli warning, begin streaming north toward Beirut. April 12 Syrian military post hit on highway near Beirut airport; Syrian soldier killed in Beirut suburb. (Syria has 35,000 troops stationed in Lebanon.) April 13 Israeli rockets hit ambulance carrying refugees near Mansoura; six killed. Israeli gunboats begin blockade of ports of Beirut, Sidon and Tyre. April 14 Israeli raid hits electrical power substation at Jamhour, a Beirut suburb. April 15 Israeli raid hits power station at Bsaleem, Beirut suburb. April 16 Ein Hilweh, near Sidon, rocketed. April 17 More villages south of Tyre attacked. April 18 Israeli rockets hit U.N. peacekeepers' base at Qana; about 100 Lebanese refugees are killed and more than 100 wounded. Apartment building hit in Nabatiyah Faawqah, killing family of eight, three others. Israeli missile boats begin naval bombardments of coastal highway. Israel says it will stop attacks if Hezbollah does. April 19 Israel moderates air raids over Lebanon, and cross-border artillery barrages. April 20 Israel steps up attacks following more Hezbollah rockets on northern Israel. Israeli gunboats shell highway north of Sidon. CASUALTIES SINCE APRIL 11 LEBANON: About 150 people killed; more than 300 wounded. 300,000 to 400,000 driven out of their homes in southern Lebanon. ISRAEL: No fatalities; about 50 wounded. Dozens of families have left northern Israel, or wait out Hezbollah attacks in underground shelters. CAPTION: Brig. Gen. Giora Inbar, commander of Israeli troops in southern Lebanon, briefs reporters on the roof of his command post in Tel Nahas.