Three rounds of UN sanctions on Iran have not changed its policy and there is no reason to think that a fourth will be any different.
The impact of sanctions was summed up by Mark Lyall Grant, the British delegate at a Security Council meeting in March.
He said: "While existing measures have had some effect, they have not yet led Iran to change course on its nuclear activities.
"That is clear from the most recent report of the International Atomic Energy Agency (IAEA), whose charge-sheet against Iran is getting longer with each report," he said.
"It reinforces our fears that Iran is acting duplicitously and illegally."
The "yet" in the first sentence is the usual triumph in diplomacy of hope over experience.
There are two reasons why sanctions have failed.
The first is that the sanctions are not targeted against Iran's vital economic interests, and Iran has developed systems of evasion in any case.
The second is that the Iranian government is more than willing to absorb the limited economic effect they have in favour of the greater political benefit it gets from continuing with its nuclear activities.
Indeed, sanctions are another flag around which to rally its supporters.
The first three rounds of sanctions (Security Council resolutions 1737, 1747 and 1803) were aimed at:
- Stopping the supply to Iran of material useful in its nuclear and missile programmes and potentially dual-use items
- Limiting the supply of major weapons systems to Iran (but on a voluntary basis)
- Banning financial dealings with Iranian banks and individuals connected with the nuclear and missile programmes
- Enabling the search of suspected Iranian ships and planes carrying banned cargo
The new resolution is expected to:
- Prevent the supply of heavy weapons, including helicopter gunships and missiles to Iran
- Tighten up the ban on dealings with Iranian banks and individuals
- Enable states to search any suspect ship or plane
The problem is that the really hard-hitting sanctions have been avoided.
The most damaging measures would be to stop the sale to Iran of finished petroleum products and to ban investment in its oil and gas industries.
Despite its wealth of oil deposits, Iran cannot refine enough products for its own use.
However, both China and Russia have huge investments in the Iran energy field and neither wants to have these affected.
They would also have the most impact on ordinary people. So these measures have not been used.
No wonder Russian Prime Minister Vladimir Putin has pronounced himself satisfied that Russian interests have been protected.
Russia has also, it appears, preserved the right to sell anti-aircraft missile systems to Iran.
A much-talked about ban on re-insurance, a vital means of supporting trade, has been watered down to a voluntary measure.
Will Iran budge?
Iran has also managed to work round the systems already in place.
In April, a Washington-based monitoring group, Iran Watch, reported that the Republic of Iran Shipping Lines (IRISL) had undertaken "a large-scale re-labeling of its ships, giving them new names, new managers, new 'owners' - in short, new identities".
"The US blacklist has not kept up with these changes, so it is being circumvented by Iran with relatively little effort," Iran Watch said.
The New York Times followed this up with a detailed account of how it has been done.
The result of all this is that Security Council sanctions have tended to be the lowest common denominator in which the important thing is to preserve the unity of the council.
Yet the more unity there is, the fewer the sanctions.
This is why the US Congress has prepared measures of its own, in addition to the blanket trade ban the US imposed in 1995.
The key measure this time would be to punish oil companies trading with Iran by preventing them from doing business in the US.
Whether this would deter China, which needs oil and gas from anywhere it can find them, and Russia, which does not see Iran as a strategic threat, is doubtful.
It may well be that pressure by the US and the EU, under which individual companies simply stop trading with Iran, might have more economic effect than the UN.
But, again, will Iran budge?