France's Maginot line was sidestepped by German forces early in the second world war, rather than the first as we stated in the article below. This has been corrected.

The new age of the wall has begun. Ramparts and stone fortifications, regarded until recently as national relics and tourist attractions, are back with a vengeance in the "global war on terror".

The White House indicated yesterday it would press ahead with building a three-mile wall between Sunni and Shia neighbourhoods in Baghdad, despite the objections of the Iraqi government.

The US insists it will be a temporary barrier, made of moveable sections. But each giant concrete segment weighs 6.3 tonnes and recent history suggests that once such substantial barriers go up, it takes a lot to take them down. In Belfast, the "peace walls" are still there, years after the guns fell silent.

When the Berlin Wall first went up in August 1961, East Germany portrayed its construction as an emergency measure but it grew into a concrete behemoth that divided Europe for 28 years. When it was finally brought down in 1989, it seemed for a while as if such barriers had had their day as a means of resolving political problems.

The Baghdad barrier is just the latest of a string of fences, screens and other such engineering solutions to the quest to keep people apart, for their own protection or for the benefit of rulers.

The conflict resolution business is in decline and the concrete industry is taking its place. Nowhere is that more evident than in the Palestinian occupied territories, where peace initiatives have taken a back seat in recent years to the construction of a "separation barrier" that has been built around the West Bank to match the one enclosing the Gaza Strip since 1994.

Israel insists the barrier is serving its purpose, sharply reducing the incidence of suicide bomb attacks. Palestinians who live in the West Bank look on the ramparts as a prison wall, designed to make the fragmentation of their ancestral lands an established fact.

The West Bank barrier and Baghdad's wall are not the only solid dividing lines rising from the map of the Middle East. The United Arab Emirates is building a fence along its border with Oman. The Saudi government, meanwhile, may resume work on a half-built barrier of sandbags and pipes on its frontier with Yemen, after a three-year delay brought on by furious Yemeni objections.

The UAE wall is primarily intended to keep immigrants out, while the Saudi barrier is being built principally with terrorists in mind. But most of the walls under construction now are meant to serve both functions.

The fences built around the Spanish enclaves of Ceuta and Melilla in north Africa, and the barrier on the Mexican border proposed by the US Congress to join up existing barriers, are two such dual-purpose projects.

The walls of ancient citadels became redundant with the development of high-explosive tank shells and aerial bombing. France's Maginot Line, effortlessly sidestepped by German forces early in the second world war, signalled the end of that era.

However, in the post-September 11 world, the principal threats to national security do not come in tanks and warplanes but a single terrorist crossing a border wielding a passport.

For that reason, Thailand is proposing to build a 400-mile barrier along its border with Malaysia in the hope that it will insulate the country from the scourge of Islamist extremism.

In some places walls still serve their ancient purpose of enclosing territory and keeping enemy hordes at bay. The modern descendant of the Great Wall of China and Hadrian's Wall is the huge sand berm built by Morocco to keep Polisario guerrillas out of the slice of the Western Sahara it claims as its own. Unlike its predecessors, which relied on height and sentries to keep the enemy out, the Moroccan wall is enforced with electronic surveillance equipment and landmines, but the purpose is similar.

Not since medieval times have walls been so in demand around the world. They are even springing up among old fortifications in Europe, where the tradition of the ghetto is rising again in its birthplace, Italy.

In Padua, the city fathers have erected a wall around a district inhabited mostly by African immigrants, blamed for the local proliferation of drugs and prostitution.

Similar walls were contemplated in Milan and in the Czech Republic, both aimed at keeping Roma, or gypsies, away from the host community, but both projects were abandoned after an outcry by civil rights groups.

While such concerns have won sporadic battles, there is no doubt that walls are once more in the ascendant, propped up by communal fears of outsiders. The few years following the fall of the Berlin Wall may be seen in years to come as an exception in the broad stretch of human history, during which people briefly thought they could live unprotected by physical barriers.