The rules of warfare are changing - you can't appeal to a Warbot for mercy

In the dark, in the silence, in a blink, the age of the autonomous killer robot has arrived.

At their current rate of acceleration they will become the dominant method of war for rich countries in the 21st century.

These facts sound, at first, preposterous.

The idea of machines that are designed to whirr out into the world and make their own decisions to kill is an old sci-fi fantasy: picture a mechanical Arnold Schwarzenegger blasting a truck and muttering: "Hasta la vista, baby."

But we live in a world of such whooshing technological transformation that the concept has leaped in just five years from the cinema screen to the battlefield - with barely anyone back home noticing.

When the US invaded Iraq in 2003, they had no robots as part of their force. By the end of 2005, they had 2400. Today, they have 12,000, carrying out 33,000 missions a year.

A report by the US Joint Forces Command says autonomous robots will be the norm on the battlefield within 20 years.

The Nato forces now depend on a range of killer robots, largely designed by the British Ministry of Defence labs, privatised by Tony Blair in 2001.

Every time you hear about a "drone attack" against Afghanistan or Pakistan, that's an unmanned robot dropping bombs on human beings.

Push a button and it flies away, kills and comes home. Its robot-cousin on the battlefields below is called SWORDS: a human-sized robot that can see 360 degrees around it and fire its machine guns at any target it "chooses".

Fox News calls it "the GI of the 21st century".

And billions are being spent on the next generation of warbots, which will leave these models looking like the bulky box on which you used to play Pong.

At the moment, most are controlled by a soldier - often 12,000km away - with a control panel. But insurgents are always inventing new ways to block the signal from the control centre, which causes the robot to shut down and "die" . So the military is building "autonomy" into the robots: if they lose contact, they start to make their own decisions, in line with a pre-determined code.

This is "one of the most fundamental changes in the history of human warfare", according to PW Singer, a former analyst for the Pentagon and the CIA, in his book, Wired For War: The Robotics Revolution and Defence in the Twenty-First Century.

Humans have been developing weapons that enabled us to kill at ever-greater distances and in ever-greater numbers for millennia, from the longbow to the cannon to the machine-gun to the nuclear bomb.

But these robots mark a different stage. The earlier technologies made it possible for humans to decide to kill in more "sophisticated" ways - but once you programme and unleash an autonomous robot, the war isn't fought by you any more, it's fought by the machine. The subject of warfare shifts.

The military claim this is a safer model of combat. Gordon Johnson of the Pentagon's Joint Forces Command says of the warbots: "They're not afraid. They don't forget their orders. They don't care if the guy next to them has been shot. Will they do a better job than humans? Yes."

Why take a risk with your soldier's life, if he can stay in Arlington and kill in Kandahar? Think of it as War 4.0.

But the evidence punctures this techno-optimism. We know the programming of robots will regularly go wrong - because all technological programming regularly goes wrong.

Look at the place where robots are used most frequently today: factories. Some 4 per cent of US factories have "major robotics accidents" every year - a man having molten aluminium poured over him, or a woman picked up and placed on a conveyor belt to be smashed into the shape of a car.

Former Japanese prime minister Junichiro Koizumi was nearly killed a few years ago after a robot attacked him on a tour of a factory. And remember, these are robots that aren't designed to kill.

Think about how maddening it is to deal with a robot on the telephone when you want to pay your phone bill. Now imagine that robot had a machine gun pointed at your chest.

Robots find it almost impossible to distinguish an apple from a tomato: how will they distinguish a combatant from a civilian? You can't appeal to a robot for mercy; you can't activate its empathy. And afterwards, who do you punish? Marc Garlasco, of Human Rights Watch, says: "War crimes need a violation and an intent.

A machine has no capacity to want to kill civilians. If they are incapable of intent, are they incapable of war crimes?"

Robots do make war much easier - for the aggressor. You are taking much less physical risk with your people, even as you kill more of theirs.

One US report recently claimed they will turn war into "an essentially frictionless engineering exercise".

As Larry Korb, Ronald Reagan's assistant secretary of defence, put it: "It will make people think, 'gee, warfare is easy'. If virtually no American forces had died in Vietnam, would the war have stopped when it did - or would the systematic slaughter of the Vietnamese people have continued for many more years?" There is some evidence that warbots will also make us less inhibited in our killing.

When another human being is standing in front of you, when you can stare into their eyes, it's hard to kill them.

When they are half the world away and little more than an avatar, it's easy. A young air force lieutenant who fought through a warbot told Singer: "It's like a video game [with] the ability to kill. It's like, freaking cool."

When the US First Marine Expeditionary Force in Iraq was asked in 2006 what kind of robotic support it needed, they said they had an "urgent operational need" for a laser mounted on to an unmanned drone that could cause "instantaneous burst-combustion of insurgent clothing, a rapid death through violent trauma and more probably a morbid combination of both".

The request said it should be like "long-range blow torches or precision flame-throwers". They wanted to do with robots things they would find almost unthinkable face-to-face.

US army staff sergeant Scott Smith boasts warbots create "an almost helpless feeling. It's total shock and awe." But while terror makes some people shut up, it makes many more furious and determined to strike back.

The Lebanese editor Rami Khouri says when Lebanon was bombarded by largely unmanned Israeli drones in 2006, it only "enhanced the spirit of defiance" and made more people back Hezbollah.

Is this a rational way to harness our genius for science and spend tens of billions of dollars? The scientists who were essential to developing the nuclear bomb - including Albert Einstein, Robert Oppenheimer and Andrei Sakharov - turned on their own creations in horror and begged for them to be outlawed.

Some distinguished robotics scientists, like Illah Nourbakhsh, are getting in early and saying the development of autonomous military robots should be outlawed now.