AN ANTENNA, towering a quarter of a mile above the New York sidewalk, sinks inexorably into a pall of smoke, a defiant gesture of a once proud building, smote by the ruthless hand of terrorism.

One World Trade Centre followed its hapless sibling to the grave after succumbing to stresses and strains unimaginable to the engineers who created the towers 30 years ago.

They were the pride of a nation and the tallest buildings in the world when they were built in 1972 and 1973. They were symbols of American decadence. They were western capitalism personified. They were monuments to a nation's pride and prosperity. New York's skyline will never be the same. Neither will the American people.

One World Trade Centre stood 1,368ft tall, just 6ft higher than its twin. It remained the fourth tallest building in the world after the Petronas twin towers in Kuala Lumpur, the Sears tower in Chicago and the Jin Mao tower in Shanghai.

When the towers crumbled, millions of tons of glass and concrete showering Manhattan, so did the American psyche, so long considered untouchable.

They had towered above the offices of the Wall Street area, testament to New York's place at the epicentre of the world economy. They drew millions of visitors a year, attracted to the highest outside observation deck in the world on the 107th floor and the "Best Restaurant in the World" called Windows on the World atop 1WTC.

Inside were thousands of mundane offices, five million square feet to be precise, workplaces to 50,000 members of staff. Morgan Stanley Bank was the biggest tenant occupying 50 floors. In the basement were shops and restaurants, a fountain plaza where hordes enjoyed their lunch.

The revolutionary structures, each 110 storeys high, were built to withstand tremendous pressures and had already survived an earlier terrorist bombing. The attack, by Muslim fundamentalists, in February 1993, ripped apart three parking levels, killing seven and injuring more than 1,000.

This time it would not be so lucky. The immense steel columns in the core proved no match for the forces unleashed yesterday and could not prevent them slipping down into a rising fog of dust and debris which engulfed the streets around the city's financial district.

A small plaque at the visitor centre reassured the visiting public that the towers were built to withstand a plane crash. But British consulting engineer Professor Alastair Soane points out that, while robust, there are limits any man-made structure can endure. "This was, of course, a completely abnormal situation and one which would not have been envisaged by the people who built it," he says. "The strength of the towers was enormous but they could not have been designed for aircraft strikes. There are buildings which are designed to withstand plane impacts but these are mainly for very sensitive buildings.

"The main impact on a building would be from the engines because they hold the mass of the plane, but there is also the fuel which would ignite."

Plans for a world trade facility had been under consideration for many years, but momentum gathered in the late 1950s with a site finally being chosen in 1962. The ground-breaking ceremony began four years later and the first tenants moved into one of the towers by the end of 1970, although the building had not then been completed. They were declared officially open on April 4, 1973. The towers were designed to be so tall to maximise the available space in the plaza below. They were initially expected to be just 80-90 floors high but the extra height was incorporated to make them the world's tallest buildings - a distinction they held until 1974.

They were built with steel columns with floor trusses extending out from the central core to the perimeter and were the first buildings of such size created without masonry.

Professor Soane, an acquaintance of engineer Leslie Robertson who constructed the towers, says: "These were very advanced designs for their time. There was a very large amount of space between the central core and the outer perimeter of the buildings which allowed a lot of office space."

The terrorists struck, too, at the Pentagon, the seat of command for the most powerful military force in the world, a building so massive it has its own subway station. The Pentagon, headquarters of the US Department of Defence, is the nerve centre for command and control of America's military might and widely regarded as one of the most secure and impregnable buildings in the world.

Experts say security is extremely tight at the complex, but that it lacks an air-shield because there has been no perceived terrorist missile threat.

There is a restricted air-traffic corridor around the building and civilian aircraft are not allowed to fly in the area.

Even if there had been an air defence system at the Pentagon, troops would have been reluctant to open fire on a civilian aircraft. The official Pentagon website says the building is "virtually a city within itself", housing about 23,000 military and civilian employees and 3,000 support personnel dedicated to protecting US national interests.

The Pentagon, named after its five-sided shape, was built near Washington DC during the Second World War, by 1,000 architects and many hundreds of workmen working round the clock. Completed on January 15, 1943, it is one of the world's largest office buildings with three times the floor space of the Empire State Building.

Both targets were torn apart by foreign terrorists - along with American pride and self-belief.