Sayyed Hassan Nasrallah has crossed the Rubicon.

The Hezbollah chairman who said exactly 13 years ago that his resistance movement would not cross the Israeli frontier – that it was for the Palestinians to “liberate” Jerusalem – has declared that Hezbollah has crossed the Syrian frontier. Not only that, but Nasrallah said at the weekend he would fight “to the end” to protect President Bashar al-Assad’s regime. Hezbollah, he said, was entering “a completely new phase.” He can say that again.

I was standing on a rooftop in the south Lebanese town of Bint Jbeil when I heard that promise from Nasrallah all those years ago. Hezbollah would not be advancing into Palestine. We all sighed with relief. What happened? In those days, Nasrallah appeared in person, standing amid his adoring fighters and their families. Now he lives in hiding. Is there tunnel vision at work? He said he receives letters from families begging him to let their sons fight in Syria. Coffin vision, perhaps?

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It was, of course, inevitable. Only last month, I discovered Hezbollah men “protecting” the Sayda Zeinab mosque in south Damascus. “Don’t say you saw me here,” one of them told me. A friendly soul who had pinned a picture of Nasrallah and of Iran’s supreme leader Khamenei to the wall of his office, he came from that very same town of Bint Jbeil.

Two days later, Hezbollah admitted their men were “guarding” the front-line shrine. Then they said 12 of them had been killed. I do not know if my interlocutor was among them. For several months, Hezbollah had been quietly admitting that their fighters were also “protecting” Shia villages inside Syria whose inhabitants were Lebanese. Then the bodies started to come home. One at first, then six, then they came by the dozen.

Once Hezbollah was committed to the battle for Qusayr alongside Syrian troops, a spokesman for this most efficient and ruthless militia claimed its fighters had been on their way to the shrine in Damascus but had been misdirected and found themselves in a firefight in no-man’s land. A likely story. Qusayr – just off from the highway to Latakia and the Syrian coast – is well over 100 miles from Damascus. Then 30 more bodies came home to Lebanon. So Nasrallah only said what he had to say. Much did he speak of Palestine and the al-Aqsa mosque. But his men were moving east into Syria, not south into Palestine, and history will judge Nasrallah on this speech.

He talked, of course, about the danger of “extremists” trying to overthrow Assad, claiming they were also a danger to Lebanon, that Assad’s Syria was a backbone of Hezbollah “and the resistance cannot stand with its arms folded while its back is broken”.

What he did not say was that his Shia militia was fighting Syrian Sunnis – whose co-religionists make up around 30 per cent of Lebanon’s population. Which is why the battle between the Sunnis and the Alawite Shias of the north Lebanese city of Tripoli broke out so ferociously on the day Hezbollah took up the fight for Qusayr alongside Assad’s men.

Quite simply, this is potentially the greatest danger to Lebanon’s people – not to mention the sovereignty of its sectarian state – since the 1975-90 civil war.

“If Syria falls into the hands of America, Israel and Takfiris [Sunni extremists], the resistance will be besieged and Israel will enter Lebanon and impose its will.” This is what Nasrallah said on the huge screen erected in the town of Mashgara on the 13th anniversary of south Lebanon’s liberation from Israeli occupation on Saturday night. What he meant was that if Assad falls, Hezbollah’s own political support and weaponry – originating in Iran – will come to an end. And then there will be no more Hezbollah to drive out the Israelis when they return.

And before we bellow with hollow laughter, let’s just remember that the destruction of the Islamic Republic of Iran – as a theological state created by Ayatollah Khomeini in 1979 – is currently the be-all and end-all of US and Israeli policy towards the country once called Persia. The 2006 war between Israel and Hezbollah was an attempt to destroy Iran’s Shia ally in Lebanon. The battle against Assad – a struggle supported by the US, the EU and those wonderful, freedom-loving demo¬cracies in the Gulf – is an attempt to strike down Iran’s only Arab ally. western-supported rebel battle for Syria is therefore a proxy war against Iran. No wonder Hezbollah has come clean about their involvement.

If – as Nasrallah insists – Hezbollah is really a “resistance” movement, how come it did not support the resistance against Assad? Besides, if Hezbollah is a purely Lebanese creature – and again, this is what Nasrallah insists – what right does it have to send hundreds, even thousands, of its men to fight Assad’s battles?

Officially, Lebanon has “dissociated” itself from Syria. But if fighters from its largest Muslim community have gone to fight for Assad, what is left of its claim to political neutrality? Nasrallah may be Hezbollah’s chairman, but he’s not Lebanon’s president. Which is why President Michel Sleiman warned just a day before Nasrallah spoke that Hezbollah should not allow Lebanon to plunge into a sectarian war. “How can a nation provide such a wonderful example of resistance and sacrifice,” he asked, “while promoting sectarian differences?” Good question.

In his speech, Nasrallah promised supporters “a new victory”. Macbeth couldn’t have put it better. Blood will have blood, they say.