The aptly named Samantha Power clearly takes a dim view of the United Nations. Which is a shame since she is the United States' new ambassador to the UN. But Ms Power clearly thinks that the body is not doing the job it was set up to do after the Second World War when the international community wanted a way of establishing a global consensus to act as a brake on the arrogant predations of a single powerful nation such as Nazi Germany.

An effective UN, she reasonably argues, should prevent President Assad of Syria from perpetrating the mass murder of his own citizens, whether by poisonous gases or conventional shells and bombs. Instead, it has acted as a brake only on those such as Barack Obama who want to curb the murderous intent of Assad and his cronies. "The system devised in 1945 precisely to deal with threats of this nature," says Power, "did not work as it was supposed to."

Her argument continues thus. International law is not just stopping the good guys from keeping the bad guys under control. It is allowing Russia to protect its prerogative to obstruct action against its Syrian ally. Such ineffectual international law is best ignored. David Cameron clearly believes the same thing, though he bungled his chance to get Parliament to back a military strike on Assad.

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But the Washington argument is only partially true. In its lifetime, the United Nations has been an imperfect tool in constraining the power of bully-boy nations. The UN has no big stick to wave when nations violate its resolutions. From Bangladesh in the 1970s, through Rwanda and Bosnia in the 1990s, and Darfur a decade later, to Syria today, the UN has seemed powerless to intervene in genocide and crimes against humanity.

Over the years this has provoked repeated calls for reform of the UN and its Security Council, which also is seen as reflecting the balance of power in 1945 rather than a modern era in which new powers such as Brazil and India are emerging. The trouble is there is never any consensus on how the UN should be reformed. So the most common response, among those frustrated by UN processes, has been unilateral military action. In Bosnia, Kosovo, Mali and Sierra Leone that was both morally and politically justifiable. In Iraq – and in the Russian invasion of Georgia – it was not.

There is an obvious reform that would restore the original purpose of the UN to establish international consensus against the wayward exercise of power by the mighty. It is to remove the veto of the major powers in the Security Council. The veto of Russia – and also of China, though its motives have been largely unscrutinised in recent debate – is what has thrice protected the Assad regime, since only the Security Council can authorise lawful military intervention.

But such a desirable change is deemed politically unrealistic in Washington. It would backfire on the US which has, since 1980, vetoed more than 30 Security Council resolutions critical of Israel – more than all the other vetoes by all the other Security Council members put together. Given that, Obama needs to apply political realism more broadly. No settlement for peace is possible in Syria without the agreement of Russia, Iran, Saudi Arabia and Qatar – the countries supplying the arms to both sides. Bombing Assad will not produce that. Persuading him to sign the Chemical Weapons Convention – and allowing UN weapons experts to deactivate what is now the largest stockpile of chemical weapons in the world – might.

Barack Obama has shown some political imagination in his handling of the crisis domestically. His decision to seek the backing of the US Congress – which as US Commander-in-Chief he does not have to do – was tactically astute. He now needs to demonstrate the same acuity internationally.

Despite intense lobbying in St Petersburg, the US President was unable to convince two thirds of world leaders that bombing Syria is a good idea. Indeed he left the G20 summit with the condemnation of both the UN Secretary General and the new Pope ringing in his ears. He failed to convince a single leader who was not onside beforehand. He might convince a few more if he were to wait for UN weapons inspectors to complete their reports on the August 21 chemical weapons attack. But he will need to do more than repeat bald assertions by American intelligence that Assad was to blame. Several G20 countries think it entirely credible that it might be a ruthless tactic by al-Qa'ida rebels designed to discredit Assad.

If Obama can build greater international consensus for a strike on Syria that would increase the "legitimate authority" of military action. But it would not fulfil the other criteria classically required for a just war. Along with just cause, right intention and competent authority there are requirements on last resort, proportionate force and a good prospect of success – in a situation where there is little agreement on what would constitute success.

All this is a far cry from Washington's current self-certainty and self-righteousness. It is not enough to assert, as Tony Blair did the other day, that "it's pretty clear" that Assad is the culprit. Blair was pretty clear there were weapons of mass destruction in Iraq. Pretty clear is no longer enough.

Vladimir Putin may have been bluffing last week when he asserted that Russia would not exclude the possibility of Security Council action if definitive proof was produced that Assad was responsible. But other world leaders are more open-minded, though they also need to be convinced that bombing will be the most productive response. There are other ways.

The truth is that unilateral action by an over-mighty power is as unacceptable now as it was when the United Nations first set out to create a mechanism for international consensus all those years ago. Tyrants do not always wear jackboots. They sometimes come in suits and ties, wearing patriotic badges in their lapels.

Paul Vallely is Visiting Professor of Public Ethics and Media at the University of Chester