Robert Fisk: The tree-lined bunkers that could change the face of the Middle East

It looks like a hop, skip and a jump. There's the first electrified fence, then the dirt strip to identify footprints, then the tarmac road, then one more electrified fence, and then acres and acres of trees. Orchards rather than tanks. Galilee spreads beyond, soft and moist and dark green in the winter afternoon – a peaceful Israel, you might think. And a peaceful Lebanon to the north, tobacco plantations amid the stony hills, just an occasional UN armoured vehicle to keep you on your toes. "Major Pardin says you cannot take pictures," a Malaysian UN soldier tells me. Then a second one says the same. Then along comes a Lebanese army intelligence officer and stares at our papers. "OK, you have permission," he declares, and I snap away with my old 36-frame real-film Nikon; the fields, the frontier fence, the high-tech surveillance tower on the horizon. This must be the most photographed border in the world.

Of course, the gentle countryside is an illusion. Benjamin Netanyahu and his colleagues in the Israeli government have been announcing that the only "army" of Lebanon is the Hizbollah, the Iranian-armed and Syrian-assisted guerrilla force whose bunkers and missiles north of the Litani river might just tip the balance in the next Hizbollah-Israeli war. And Sayed Hassan Nasrallah, the chairman of the Hizbollah, has been making some even more interesting threats: that his forces will "change the face of the Middle East region" if there is another war with Israel. No-one is in much doubt about what this means. The newly resurfaced Lebanese roads near the border – courtesy of Hizbollah money – suggest that someone might want to move men at high speed towards the frontier. Perhaps even to cross the border.

That's what the Israelis suspect, too – and it makes sense of Nasrallah's warning last week. The Hizbollah claimed that the 2006 war with Israel was a "divine victory" – it didn't feel that way to us in southern Lebanon at the time – yet even Israel admits it was a near-defeat for its own ill-trained soldiers. But how would Israel react if the Hizbollah managed to enter Israel itself? Israeli army commanders are talking about this in the Israeli press. A fast, dramatic spring across the frontier to the west – in the direction of Naharia, perhaps, or a grab at the settlement of Kiryat Shmona – and Hizbollah would announce it had "liberated" part of historic "Palestine". Israel would have to bomb its own territory to get them out.

This is no game. The Israeli army wants to revenge itself on the Hizbollah, which humiliated it in 2006. Nasrallah – on giant-wide screens, for security reasons – often talks as if he's the Lebanese president. Did the Israelis really think al-Qa'ida or the Hizbollah were beind the attempted killing of two Jordanian diplomats between Amman and the Allenby bridge, Nasrallah asked. No friend of al-Qa'ida, Hizbollah would have succeeded in blowing them up if it had been involved. The crowd roared its agreement.

But the threats continue. The Israeli defence minister, Ehud Barak, says that the Lebanese government will be held responsible for any future war and the Lebanese have had the usual warnings from Israel. Lebanon's infrastructure will be attacked, its bridges and highways destroyed, its villages erased. Israel, Mr Barak has been saying, was restrained in 2006 – when it attacked Lebanon's infrastructure, destroyed its bridges and highways and erased its villages. Plus ça change.

But there's a good deal of "change". Syria is being courted by the Obama administration. Its old allies in Lebanon – Druze leader Walid Jumblatt among them – are uttering honeyed words to Damascus. Indeed, Jumblatt has been meeting both Nasrallah and his old enemy Michel Aoun, and concluding that he is three-quarters of the way down the road to Damascus. And President Assad of Syria has been visiting Tehran again, to assure the Islamic Republic of his ever-loyal support.

You can see the way everyone is thinking. And here's the big question, the camel in the room. If Israel ignores Obama and attacks Iran's nuclear sites – a real aggression if ever there could be – the Hizbollah could fire rockets into Israel, perhaps even revealing its new anti-aircraft missile capacity. Hamas might join in from Gaza. Hamas is a tin-pot outfit; the Hizbollah is not. An Israeli attack on Iran will unleash Iranian military power against America. But part of that power is Hizbollah in Lebanon. This is serious business.

Over Christmas, a parcel "from a foreign country" was delivered to three Hamas officials in Beirut and blew up, killing all of them. Last week, a bomb exploded in a building in southern Lebanon owned by two Hizbollah officials, wounding three children. One of them, 11-year-old Diana Zreik, had her left leg amputated. It looks like a glance at the past, to the 1970s, when Israel posted letter-bombs to its enemies in Lebanon.

The United Nations has been complaining at the increase in Israel's overflights of Lebanese territory. The Lebanese army has been opening fire on Israeli aircraft flying over the border – useless, of course, because the Americans don't give the Lebanese army weapons that can hurt Israel – while US Senator John McCain has dropped by in Beirut to complain about the Hizbollah's weapons which, under UN Security Council Resolution 1701, are supposed to be in the hands of the Lebanese army. This is the same resolution that should prevent Israeli overflights.

And what do those overflights show? "We see Hizbollah expanding inside Lebanon and its growing influence, political and otherwise," Barak said last week. "We again wish to make clear to the Lebanese leadership that we see everything, and we will hold the parties which cause increased tension responsible... the situation can quickly deteriorate." Thank you, Israel. Especially if Israel attacks Iran.

Party of God: Hizbollah and the politics of Islamic resistance

* Hizbollah, which can be translated as Party of God, is a Lebanese, Iranian and Syrian-backed, Shia Islamist political movement with a paramilitary wing known as "Islamic Resistance". It emerged in 1982 as a small militia force with the aim of ending Israel's occupation of southern Lebanon.

The movement is now a major player in Lebanese politics with its own satellite TV network, a radio station, and a vast network of social programmes from housing to agriculture. According to Hizbollah's manifesto, the Lebanese people must be free to choose what form of government they want, but they are strongly encouraged to pick the option of Islamic government. The movement's original goal of turning Lebanon into an Islamic Republic has been abandoned.

* Sheikh Hassan Nasrallah became Hizbollah's Secretary General in 1992. A popular and charismatic figure, his face appears from billboards and hoardings across Beirut and south Lebanon. The deeply religious 49-year-old studied theology in Iran and is noted for his fiery sermons, excerpts of which are sometimes heard on mobile phone ringtones.

* Israel tried to kill Nasrallah during the month-long war it fought against Hizbollah in 2006. The conflict followed a Hizbollah attack on an Israeli army convoy patrolling the border with south Lebanon. Nasrallah now says Hizbollah will defeat Israel in any new conflict. "I promise you that should a new war with the Zionists erupt, we will crush the enemy, come out victorious and change the face of the region," he said last week.