In essence, they are economic embargoes covering a variable selection of goods and/or services applied by one country or group of countries against another, aimed at influencing the latter's behaviour.

They are as old as history; some say the first recorded instance was in 432 BC when Athens imposed punitive trade sanctions on Megara, ally of Athens' great rival Sparta. (Alas, the move helped provoke the Peloponnesian war, that would lead to the destruction of Athens as a major power. ) Sanctions are a middle option in the toolkit of international relations. They are more severe than a mere diplomatic rebuke, but stop short of the ultimate step of military force.

Do sanctions work?

Unilateral sanctions almost never do. So diverse is the world-trading system that a country which is being deprived of goods or services by one country can invariably secure them somewhere else.

The US, with its addiction to moral outrage, is the greatest exponent of sanctions, either maintaining or threatening them against 75 countries that contain 42% of the world's population. US sanctions against Iran date back to 1979. Yet this unrelenting pressure has failed to topple the Islamic regime . . . to the point that President Bush recently admitted that the US on its own was "sanctioned out, " as he demanded international action against Tehran to force a halt to its suspected nuclear arms programme. In some cases, most notably Cuba, unilateral sanctions have even been counterproductive, strengthening nationalistic support for Fidel Castro. If anyone has been isolated by Washington's 40plus years of sanctions, it has been the US, not Cuba.

What about multilateral sanctions?

These obviously have a greater impact. Not only are they harder to evade, but they send a stronger signal. Multilateral sanctions can be imposed by an ad-hoc group of countries (the sanctions equivalent of a "coalition of the willing") or, most effectively of all, by the United Nations Security Council.

Sanctions resolutions against Iraq, Yugoslavia . . . and now most probably North Korea . . . are a mark of opprobrium from the entire international community. They may not have an immediate impact, but they cannot be easily ignored.

How often have they been applied?

The Council has adopted sanctions 16 times in all . . . mostly after the end of the Cold War, when the Soviet Union, which once systematically blocked every Western initiative at the UN, no longer existed. The countries targeted, in alphabetical order, have been: (Taliban-ruled) Afghanistan, Angola, the Democratic Republic of Congo, Ethiopia, Haiti, Iraq, Ivory Coast, Liberia, Libya, Rwanda, Sierra Leone, Somalia, South Africa, Rhodesia (now Zimbabwe), Sudan, and the former Yugoslavia.

Have UN sanctions been effective?

The record is mixed. Sanctions against Slobodan Milosevic's Yugoslavia probably hastened the 1995 Dayton accords, which ended the Bosnian war. By denying high-tech imports, they certainly made it harder . . . if not impossible . . . for Iraq to rebuild its WMD programme after the 1990/91 Gulf war. They may have prodded Libya towards cooperating with the Lockerbie investigation, and then abandoning its own WMD ambitions.

The greatest claimed successes of sanctions, however, came much earlier, in Rhodesia and South Africa. But others argue they had little real effect on the racist regimes in Salisbury and Pretoria, and that these would have collapsed anyway because of internal pressures.

What are the drawbacks?

The biggest is that comprehensive sanctions can end up hurting the people they are designed to help. Iraq, the target of unrelenting sanctions between the summer of 1990 and the March 2003 invasion, is the classic example.

UN sanctions brought poverty and hardship for ordinary Iraqis . . . and possibly the death of hundreds of thousands of Iraqi children . . . while Saddam cynically manipulated the system to enrich himself and his ruling clique. Ultimately, this failure was widely advanced as a justification for the invasion.

Sanctions, moreover, can produce counterproductive displays of disunity. Saddam long played on divisions between the US and Britain on the one hand, and France, Russia and China on the other, among the five vetowielding members of the Security Council. Iran and North Korea, both now in line for UN sanctions, will take heart if Russia and China refuse to back the severe sanctions sought by the US and its allies.

What about 'smart' sanctions?

These are tailored to have the maximum influence on the rulers of a country, while minimising suffering for the population at large.

The "smart" sanctions arsenal includes travel bans on senior officials of a regime and a freeze of their overseas assets, as well as arms embargoes and the selective ban of key imports.

They can be more easily operated by a small number of countries, especially those home to the major financial markets in Europe, North America and the Far East. Evidence is growing that clampdowns on travel and overseas bank accounts do have an impact.

Can sanctions serve a purpose, even when they don't 'work'?

Sometimes. After the Soviets invaded Afghanistan in December 1979, President Jimmy Carter imposed unilateral US sanctions, including a boycott of the 1980 Olympics. Everyone knew the measures in themselves would have no direct impact, but the stern reaction sent a message that further Soviet expansion, towards Iran and Pakistan and their coveted warm water ports, would not be tolerated.

So what now?

The United States will continue to dispense punishment by sanction . . . perhaps even more than in the past, given the overstretch of its military. Petty and spiteful, the embargo on Cuba will remain, at least until either Castro dies or George Bush leaves office.

The UN seems certain to take action of some kind against North Korea and Iran, and possibly against Sudan . . . China permitting. China, the emerging global superpower has taken over the former spoiler's role of the Soviet Union. It opposes truly harsh sanctions against Iran and North Korea, and is blocking UN action to force Sudan to change policy on Darfur.

SHOULD THE UNITED NATIONS IMPOSE SANCTIONS ON NORTH KOREA?

Yes. . .

>> They would send a message that the leadership in Pyongyang could not ignore >> They would slow the further build-up of North Korea's weapons programmes, both bombs and missiles >> If China were on board, the North would be deprived of the support of its most important ally

No. . .

>> Sanctions would have little impact, given the military self-reliance of the regime >> North Korea would be even more desperate to export dangerous military technology, to gain foreign exchange >> Sanctions would only increase the hardship of the impoverished North Korean people