By Paul Reynolds
World affairs correspondent, BBC News website
Robert Mugabe: Untroubled so far by sanctions
The proposed sanctions on Zimbabwe rejected by the UN Security Council after vetoes by Russia and China on Friday are the latest examples of a diplomatic and economic weapon that has a distinctly mixed history of success and failure.
Sanctions sometimes have the appearance of being more about making those who impose them feel better than making those at whom they are aimed change their minds.
In the case of Zimbabwe, for example, the British and American-led proposal was that the top 14 people in the country's political and security apparatus should not be able to travel abroad and should have their assets abroad frozen. An arms embargo was also proposed.
However, those prepared to use force to maintain their positions are hardly likely to worry about not being able to travel and as long as they have got their money safely back home, they will continue to live well. Arms embargoes are often ineffective.
In any event, the proposals failed to convince Russia and China, so they were not adopted.
Often the problem with sanctions is they fail to have the effect intended.
Sanctions are an easy diplomatic weapon for governments to reach for. They have the advantage of helping to quieten domestic critics, even if they do not actually make much difference.
And these days those domestic critics, often in the form of powerful non-governmental lobby groups, have to be listened to and usually placated.
Zimbabwe itself, under its colonial name Rhodesia, provides an example from an earlier time.
From an earlier age: Rhodesian leader Ian Smith survived years of sanctions
The then British Prime Minister Harold Wilson said that sanctions on the white minority rulers would change things in "weeks not months". It took 12 years and a guerrilla war - led by Robert Mugabe - to effect that change.
For many years, South Africa was under an arms embargo. This encouraged it to develop its own arms industry. In the end it was able to export arms itself.
The question of whether sanctions and isolation convinced the apartheid rulers that they could not go on is still an issue for debate.
Or was it the realisation that apartheid was simply not working that made the difference?
The Cuban example
The United States has imposed trade restrictions on Cuba in various forms since 1960. It did not in any way weaken the position of Fidel Castro, who has now retired an elderly man.
On the contrary, it gave him an excuse for poor economic performance by being able to blame someone else.
Cuba could be an example of sanctions having the opposite effect to the one required. Another might be the oil embargo imposed by the US on Japan in 1940 after its invasion of southern Indochina.
This might have increased Japan's sense that it needed to acquire its own supplies by attacking oil-rich counties of South East Asia.
Iran and sanctions
Likewise, in Iran, broad American trade restrictions came in after the US hostages were seized at the US embassy there in 1979.
They have been maintained since. Iran, for example, has not been able to buy aircraft from the US.
But the revolutionary leaders of Iran have not been deterred by these sanctions and very much remain in power today. They often use US hostility as a banner around which to rally support.
Limited sanctions against Burma have not shifted the ruling junta there.
The Arab embargo on Israel has not changed Israeli policy.
Targeted sanctions the trend
Aware that broad-based sanctions can be pointless, policymakers have turned more and more in recent years to so-called "targeted" sanctions.
These aim at specific items needed by the target government or at particular people in that government. They avoid the criticism that it is the ordinary people who bear the brunt of sanctions.
Iran is currently the subject, not only of wide US trade restrictions, but of targeted UN sanctions over its nuclear activities. Specific people, businesses and organisations are targeted for boycott and isolation.
This does seem to have some effect. According to the US Under-Secretary of State William Burns, Iran has been hampered by restrictions on trade in missile technology.
He told a congressional committee: "As a direct result of UN sanctions, Iran's ability to procure technology or items of significance to its missile programs, even dual-use items, is being impaired."
What really counts?
It is also arguable that sanctions maintained on Saddam Hussein after his army was ejected from Kuwait in 1991 made it impossible for him to get technology from abroad, thereby leading to his abandoning any hopes of developing nuclear weapons and weapons of mass destruction.
But his failure in war and UN inspections might have been even more decisive.
And did Libya change course and give up any nuclear ambitions because it was under sanctions following the Lockerbie bombings? Or did it realise that it might be attacked by the US, which, with the UK, had found hard evidence of nuclear smuggling?
Another example of targeted sanctions was the 1974 Jackson-Vanik amendment in the US. It aimed at getting the freedom to emigrate for Soviet Jews by denying full trading rights for any country that did not allow free emigration.
The White House says the amendment was "an extraordinary success in securing freedom of emigration in the Soviet Union and its successor states".
Tit for tat
Sometimes sanctions descend into tit-for-tat symbolism.
President Jimmy Carter boycotted the Moscow Olympic Games in 1980 following the Soviet intervention in Afghanistan at the end of 1979. In retaliation, the Soviet Union stayed away from the Los Angeles Games in 1984.
All this had no effect on the situation in Afghanistan, in which the fighting was the decisive element.
Sanctions are not new in international politics, of course.
Way back in about 432BC, the Athenians stopped the people of next-door Megara from trading throughout the Athenian empire. It was over some dispute about trespassing on sacred land.
The result was that Megara's ally, the Spartans, were mightily displeased. Historians debate whether the Spartans would have gone to war in any case but they did.
The sanctions do appear to have been counterproductive, to say the least. An old story.