When Hussain Alawieh used to take tourists to Beaufort Castle, they would marvel at the view. The ancient hilltop fort, captured nearly 1,000 years earlier by Crusaders, still offered the same sweeping panoramic views of south Lebanon and the Litani River that empires fought over for a millennia.

On Sunday, the view from the castle was obscured by white phosphorus smoke, the toxic incendiary munition providing a smoke screen for advancing Israeli soldiers. Out of the fog rose an Israeli flag, and the castle, for the first time in 26 years, was once again conquered.

In the age of drones and surveillance blimps, the value of the ancient hilltop fort is diminished. But to both Israelis and Lebanese, its capture carried psychological weight in a conflict that for six weeks had ground to a deadlock.

“The raising of the Israeli flag and the flag of the Golani Brigade above the castle caused a shock to me and to all southerners and Lebanese people,” said Alawieh, a tour guide based in south Lebanon.

The castle, Alawieh explained, was a symbol of steadfastness and of resistance in south Lebanon. Its thick stone walls helped it survive Israeli aerial bombing in the 1980s when it was used as a base by the Palestine Liberation Organisation, and again when Israel carried out a detonation in the castle upon its withdrawal in 2000.

“Raising the Israeli flag above it is intended to send a message of psychological domination and defeat to the population, conveying that the ‘sites you considered impregnable have fallen’,” said Alawieh.

The capture of the castle came as Israel’s invasion of south Lebanon lurched forward once again. The pace of the war in Lebanon had slowed since a supposed ceasefire on 17 April. With much of south Lebanon declared a no man’s land by Israel, it was impossible to tell what was happening on the battlefield.

Last week, what was a low-intensity war suddenly accelerated, with Israeli warplanes killing at least a dozen people a day, and Israeli soldiers once again marching forward.

Beaufort Castle was the most tangible marker of Israel’s progress, both to Israelis and Lebanese. Netanyahu, facing pressure from his domestic political rivals, happily announced that Israel was deepening its invasion in Lebanon.

To the Lebanese, the sight of the Israeli flag over the castle brought back memories of its 18-year occupation of south Lebanon starting in 1982.

“Of course, it brought me back to the occupation. We went back to 1986, 1987, and 2000. It brought back memories of those painful days,” said Fouad Fatimi, the mayor of Arnoun, where the castle is located.

Arnoun had been emptied out in the weeks before its capture, as Israeli airstrikes pounded the town and its surroundings. Fatimi had recorded a phone call he had received last month from an Israeli officer telling him that residents must leave the village.

Israeli soldiers arrived to an empty village and a castle undefended. The Israeli military drove the point home; it shared footage of its soldiers striding up the castle’s steps set to a song by Lebanon’s most famous singer, Fairuz, entitled Waynun, its chorus repeating: “Where are they? Where are they?”

As Israel’s soldiers patrolled the castle, its warplanes dropped bombs on south Lebanon, leaving little time to absorb the new loss of territory. The city of Tyre was pounded with airstrikes on Sunday, leaving smoking craters where residential buildings had once stood. Entire neighbourhoods of one of south Lebanon’s oldest and most populated cities were covered in rubble and immense plumes of smoke rose above its homes.

The city’s civil defence withdrew from the city before the bombing on Sunday. The Israeli military had called them and demanded they evacuate. They returned on Monday, establishing a new headquarters in the city’s Christian quarter, where Israel had not yet bombed, according to the head of Tyre’s civil defence, Ali Safieddine.

Israel’s campaign expanded further on Monday, with Beirut once again coming under threat – the last feature of a ceasefire which had until now left the country’s capital largely untouched. On Monday morning, Israel’s defence minister, Israel Katz, said the military would once again start striking Beirut.

Roads leading out of the southern suburbs were soon choked with cars heading north, as people fled their homes after returning home just six weeks earlier. The streets of Beirut were filled with the sounds of car horns as they sought to escape.

WhatsApp chats contained messages of resignation. “Here we go again,” one resident of the southern suburbs sent to a group chat. Others desperately inquired if anyone knew of empty apartments for families displaced anew.

Both the Lebanese government and Hezbollah issued condemnations of the escalation, but neither seemed to able to stop it.

“[The resistance] has never claimed to prevent invasion or occupation of territory, nor has it claimed to posses an armament balance,” said Hassan Fadlallah, a Hezbollah MP, on Sunday, adding the group would work to prevent the Israeli military from “consolidating control” over the areas it has already occupied.

Unable to stop the advancing Israelis, many Lebanese could do little else but look towards the castle’s history as a symbol of hope that they may one day return to their villages.

“Seeing the castle once again covered by the flag of occupation was regarded as a deep wound to our national identity,” said Alawieh. “But I see this presence as temporary, looking at the history of the castle, which has cast out all invaders and occupiers before.”