Can a tree start a Middle East war? It almost did yesterday.

That such a question can be asked is a symbol of the incendiary state of the region, the mutual distrust of Arabs and Israelis, and the dangerous border of southern Lebanon which was – as so often – drenched in blood yesterday, the blood of three Lebanese soldiers, an Israeli lieutenant-colonel and a Lebanese journalist outside an otherwise nondescript village called Addaiseh.

And after the tank shells, Israeli helicopter missile attacks, Lebanese machine-gun and rocket-propelled grenade fire, the UN called on both sides to "exercise restraint" and the battle died down under the cold eyes of a Spanish UN battalion and a few soldiers from Malaysia.

But this comes after a tripartite Arab summit in Beirut, mysterious rocket attacks on the borders of Jordan, Israel and Egypt two days ago, a claim by the Lebanese Hizbollah that the UN inquiry into the murder of ex-prime minister Rafiq Hariri was an "Israeli project", and the discovery – on Monday – of yet another alleged Israeli spy in the Lebanese telephone network.

But back to the tree. It was a miserable, scrawny thing, probably a spruce and – after a 46-degree heatwave in Lebanon – its foliage blocked the Israeli security cameras on the Israeli-Lebanese border near Addaiseh. The Israelis decided to use a crane to rip it out. But there's a problem. No one is exactly sure where the Israeli-Lebanese border is.

In 2000, the UN drew a "Blue Line" along what was – in those long ago, post-Balfour days – the frontier between the French mandate of Lebanon and the British mandate of Palestine. Behind it, from the Lebanese point of view, stands the Israeli "technical fence", a mass of barbed wire, electrified wires and sandy roads (to look for footprints). So when the Lebanese army saw the Israelis manoeuvre a crane up to the fence yesterday morning, they began to shout at the Israelis to move back.

The moment the crane's arm crossed the "technical fence" – and here one must explain that the "Blue Line" does not necessarily run along the "fence" – Lebanese soldiers opened fire into the air. The Israelis, according to the Lebanese, did not shoot in the air. They shot at the Lebanese soldiers.

Now for the Lebanese army to take on the Israelis, with their 264 nuclear missiles, was a tall order. But for the Israeli army to take on the army of one of the smallest countries in the world was surely preposterous, not least because Army Day had been attended by the president of Lebanon, Michel Sleiman, in Beirut only two days earlier – when he ordered his soldiers to defend their frontier.

At about this time, Al-Akhbar newspaper's local correspondent Assaf Abu Rahal turned up in Addaiseh to cover the story. And a little time later, an Israeli helicopter –apparently firing from the Israeli side of the border (though that has yet to be confirmed) – fired a rocket at a Lebanese armoured vehicle, killing three soldiers and the journalist.

Lebanese troops, on orders from Beirut, fired back and killed an Israeli lieutenant-colonel. Hizbollah, the Iranian-paid Shia militia, which was not involved in the battle, announced his death five hours before the Israelis confirmed it; their information apparently came from an Israeli soldier using a mobile phone. It was top of the headline news on Hizballah's Al-Manar television station.

All afternoon, the Israelis and Lebanese abused each other as aggressors. Israel said the whole thing was a misunderstanding. Saad Hariri, Lebanon's prime minister and Rafiq's son, was on the phone to President Hosni Mubarak of Egypt, denouncing "Israeli violations of Lebanese sovereignty", while Israel said it was taking the whole affair to the UN Security Council. "Israel views the Lebanese government as responsible for this serious incident and is warning of ramifications if the violations continue," a spokesman said. Because of a tree? Of course, the Israelis would like to have a file of "incidents" before the next Hizbollah-Israel war, when they have promised to smash up Lebanon's infrastructure for the sixth time in 32 years – on the grounds that Hizbollah is now represented (as it is) in the Lebanese cabinet.

And all this while President Ahmadinejad of Iran – one of Hizbollah's sponsors – claims he wants face-to-face talks with President Obama over Iran's nuclear programme, and when the International Crisis Group has just come out with a new report warning that the next Israel-Lebanese war will be more violent than ever.

Yet the Israelis used tank shells and helicopters yesterday; the Lebanese army used rocket-propelled grenades and heavy machine-gun fire in the opposite direction. Briefly, Lebanon's much-abused mobile-phone system almost collapsed. Not because of Milad Ein, the alleged spy who worked for the Ogero landline communications company. But because everyone wanted to know if another war was about to start. Because of a tree.

An Explosive border

"Exceptionally quiet and uniquely dangerous" was how one group of experts yesterday described the border dividing southern Lebanon and northern Israel.

An uneasy calm has prevailed over one of the world's most combustible political boundaries since the 2006 war waged by Israel against Hizbollah. But the region, littered with landmines and patrolled by Lebanese troops along with 13,000 UN peacekeepers, remains as tense and volatile as ever.

The Brussels-based think-tank, International Crisis Group, warned yesterday that the political roots of the 2006 crisis remain unaddressed and that another war would be more devastating than the last.

Hizbollah, the Iran-backed militia movement that fought Israel in 2006, took no part in yesterday's skirmish, but its leader, Sayyed Hassan Nasrallah, said his group would react if the Lebanese army was attacked again.

"The Israeli hand that targets the Lebanese army will be cut off," he said.