In a grand manor house in northern Lebanon, eight men, all of them well-to-do professionals, had gathered to hear an important visitor talk about the war.

Their guest was late; the going had been heavy across the mountain from Beirut, north up the Bekaa valley and finally west along the flat, spotless, Iranian-made highway that leads to Hermel. Arriving from the bitter chill of a winter evening, he eased into a warm living room where the expectant men edged forward, addressing the new arrival by a nom de guerre widely known throughout Hezbollah, the powerful militant group he had joined more than 20 years ago.

“This is a war not just against us, but against humanity,” he said. “And it is one that we will win.”

He was referring to the war to the east in Syria, a conflict in which Hezbollah has admitted playing a significant role, rallying to the cause of the Syrian army in its protracted battle against the opposition forces and Sunni Islamist groupings ranged against it.

Speaking carefully and deliberately, the commander, whom the Guardian agreed not to name, initially stuck to the official script that characterises Hezbollah as reluctant saviours of a beleaguered nation hemmed in by extremist Sunni militants on one side and by Israel on the other. In nearly three years of insurrection and war in Syria, it has been difficult to hear anything else from a Hezbollah official.

But over two increasingly unguarded hours, the commander strayed on to themes rarely covered: the regional impact of the group’s role in Syria, the intensity of the fighting and the performance of the Syrian army, which not long ago had been fighting a losing battle to retain control of the country. Those foregathered listened intently. All broadly supported the fight against the Syrian opposition, even if they differed on the virtues of Bashar Al Assad.

“They fight well. It is not fair to them to say that they are not taking the lead,” he said of the battle-worn military regime. “They are there and they are fighting. They have lost 30,000 men. That is not an army that isn’t fighting. We are there giving advice and in some cases tactical leadership. We do not take a lead role.”

Fifteen kilometres north-east, the ruins of the Syrian border town of Qusayr tell a different story. In May, Hezbollah stormed the town from the south, achieving in three weeks what the army it supports had been unable to do in two years. Syrian tanks and troops took blocking positions to the north and east.

The Qusayr battle cost Hezbollah 112 men. It was, however, defining for a different reason: it marked the first time that the group’s leader, Hassan Nasrallah, had been prepared to reveal that his members were fighting in Syria.

Throughout the discussion, the commander labelled all members of the opposition as takfiris. Pressed on whether he believed any opposition fighters remained committed to the uprising’s original goals of reorienting power within Syria’s current borders, he said: “If there were any mainstream revolutionaries back then, there are very few now.“The battle is intense. The takfiris are committed. They want to destroy Syria and we will not let them,” said the Hezbollah leader.

Despite murmurings of unease in parts of Lebanon’s Shia heartland, he said Hezbollah and its supporters resolutely supported the group’s involvement in the war. “It is an extension of the ongoing war [with Israel],” he said. “The enemy wears a new cloth. They may not be doing all of this themselves, but their interests are being served.”

Asked why it had taken more than two years for Nasrallah to acknowledge the group’s intervention, he said: “There was a process needed. People are absolutely committed to the reality now because they know it is one and the same hand.

“We started around the Sayeda Zainab mosque [a revered shrine near Damascus], then moved to the border villages, then Qusayr. There are members fighting throughout the country, but not in huge numbers.”

On the other side are a mix of Syrians fighting to oust Assad and replace him with another leader, and jihadists who see the insurrection as means of re-establishing a caliphate in the area and a fundamentalist Islamic society.

Fighting in both areas has been intense over the past week, with more than 420 people reported to have been killed in eastern Aleppo.

Here, only 15km from one of Hezbollah’s main strongholds, the world view also changes suddenly.

At the first Lebanese army checkpoint heading south, several soldiers stopped our car and asked if we had a place for one of them. “I’m deserting,” a 19-year-old Sunni Muslim conscript said. “I’ve had enough. Another rocket [from Syria] just landed. I want to go there and fight.”

Echoing the words of the Hezbollah leader, he said: “This is a war that we cannot lose. We will win, whatever the cost.”—By arrangement with the Guardian