The most extraordinary spectacle of the past week has been the apparent desire of the US Congress to pronounce as genocide the 1915 massacre of Armenians by Turks, even though there is nothing more provocative to Turkey, and no worse time at which to do it.

Why take up an historic cause with such passion? And why now, when the most precarious planks of US foreign policy rest on already fraying relations with Turkey? It is not just the Bush Administration that has asked Nancy Pelosi, the Democratic Speaker of the House of Representatives, to desist. Eight former secretaries of state, of all political hues, have signed a petition calling for the same.

The House’s move reflects the power of the Armenian lobby, which has cherished this cause above anything more modern. It found a moment when Congress was receptive; Democratic leaders have been looking for ways to attack the values and tactics of the Administration’s foreign policy. But this is a bad way to do it.

Of course, Turkey might just mark down Congress’s gestures as an example of the contradictions thrown up by the separation of powers in US government. It could say that alleged actions by the Ottoman empire, a decade before the founding of the modern republic of Turkey, are none of its concern. But it won’t. The pity is that this frivolous move could have serious consequences: for stability in Iraq, for US forces there, for Nato, and as the markets have noticed, for the price of oil.

This was never a subject on which Congress should have set to work. It has no business pronouncing on an historical debate on which there is still enormous controversy. The massacre occurred a year into the First World War when millions of Armenians, who had fled the expansion of Russia and its satellites, tried to set up an independent state in Anatolia. The Armenian lobby says that Turks killed 1.5 million of them; Turkey denies that the number was that high, and says that many Turks died too. Documents supposed to record the atrocities have been disputed as forgeries.

Now the Committee on Foreign Affairs of the House of Representatives has passed a resolution “finding” the massacre to be a genocide. It was not unanimous – 27 in favour, 21 against – but 226 of the 435 members of the House helped to write the resolution. Next month it will be reviewed by Congress.

This reflects the power of the million people of Armenian extraction in the US. In 15 years, the US has given Armenia more than $1 billion of aid.

House Democrats have said that the resolution stands for a wider commitment to uphold American values and to use foreign policy for preventing genocide. But why pick on a century-old dispute, rather than the affront represented by, say, Guantanamo Bay or genocide in Darfur? It is perverse to think that by taking a stand about long-past events you will uphold values that more recent actions have jeopardised.

The advocates of this backward-looking condemnation are also open to the charge that they are recklessly endangering honourable goals of current policy.

The US has spent weeks pleading with Turkey not to send troops over the border with Iraq in pursuit of rebels. It may well fail. Any incursion would inflame the only part of Iraq that is at peace – and jeopardise US troops, who get more than a third of their supplies over the border.

There is plenty for Congress to do in challenging current US foreign policy. The 1915 massacre would be better left to historians.

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