First, the fundamentals.

Those countries that opposed war with Saddam certainly believed him to be a threat to regional security.

But, crucially, they believed that the threat he posed was containable. It was for this reason that they believed war an excessive response.

The nature and magnitude of the Iranian threat is entirely different. Iran has an advanced nuclear power programme, and its claims that this is for purely peaceful purposes are no longer believed by anyone. It is now moving towards becoming an uncontainable threat. So grave is the matter, and so repeated Iranian transgressions, that the UN's 35-member nuclear watchdog gave up trying to make it comply with its commitments earlier in the year and sent the matter upstairs to its highest authority, the Security Council.

The manner in which Iran conducts itself is also very different from the Iraqi dictatorship. Iranians are infinitely more skilful and subtle in statecraft than Saddam's ham-fisted and fear-stricken minions.

These skills mean that the Iranians are far less likely to make the sort of miscalculation that caused Saddam to believe he would not be overthrown.

Of the other main actors in this drama, their conduct has, for different reasons, changed since the Iraq crisis. The Europeans are at one in how they view the Iranian nuclear threat, in stark contrast to their different assessments of Saddam's capacity for menace. This can be seen in their cohesion in dealing with the matter. Ireland and 21 other EU members have (unusually) been willing to let the big three . . . France, Germany and the UK . . . take the lead in mounting the bloc's most concerted common diplomatic effort ever.

US conduct now and then is even more different. It has reverted to the more conventional approach of doing business, using multiple means and exercising a tactical flexibility absent in the run up to the Iraq war. But that is not to say that Americans have converted to pacificism.

The US deems the threat to its security of a nuclear armed Iran unacceptable. It will take whatever measures necessary . . . including the resort to force . . . to prevent such an outcome, even if the consequences in the region would probably be appalling. In short, if it boils down to a choice between the unacceptable and the appalling, the latter will win out, as it always must.

There is yet another difference between the current crisis and Iraq. If it does come to war, the nature of the conflict will be more similar to the first Iraq war (in 1991) than the second. A targeted attack to knock out nuclear facilities would be the (limited) objective, rather than regime change, not least because post-2003 Iraq has disabused all but the most ardent ideologues of the belief that stable democracies can be socially engineered.

Mercifully, though, conflagration is still some way off and diplomacy could still triumph. The Europeans continue to plug away doggedly . . .

they offered Iran yet another package of incentives this week. But, yet again, this was spurned, and the Euro route looks all but at an end.

Sanctions are now looking increasingly likely, even if agreeing a regime will not be straightforward. Russia is the pivotal power here, but it is hard to predict how it will behave. In the four years since the build-up to the Iraq war began, the former superpower has changed. Emboldened by the clout its huge energy resources are giving it, it is asserting itself more aggressively in international affairs and is at pains to make sure noone takes its support for anything for granted.

This new-found assertiveness has already seen the thwarting of efforts at the Security Council to agree a sanctions regime, and if it does so again, focus on bi-lateral relations between the US and Iran will intensify.

Increasingly it appears that a "grand bargain" between the two offers the only real prospect of a solution. This would involve, at its core, Iran giving verifiable guarantees about its nuclear programme in return for the US giving the Islamic republic commitments on regional security. However rational a solution this appears, lamentably it may not be achievable.