William Horsley BBC European Affairs Correspondent

The European Union's Athens summit was supposed to be about enlargement.

It was called so that all 15 leaders of the current member states could join those from the 10 newcomers, mostly from eastern Europe, in signing the accession treaty that will expand the EU to 450 million people.

A spectacular ceremony will be held on Wednesday beside the Acropolis, a symbol of the world's first democracy. But deep divisions over Iraq have cast doubt on the goal of some leaders to make the EU a global power with a single, clear foreign policy.

All smiles - but tough talking lies ahead

United Nations Secretary General Kofi Annan has travelled to Athens to try to bridge the differences over setting up an interim authority to govern Iraq, and over lucrative commercial contracts there.

Last weekend for the first time the US and the UK, which joined forces to overthrow Saddam Hussein's regime, seemed to find important common ground with opponents of the war like France and Germany.

Finance ministers of the G7 leading industrial states agreed in principle on the need for a multilateral effort to help Iraq, and a further UN Security Council resolution on Iraq's reconstruction.

But any progress is likely to be slow.

Arguments are inevitable over key issues like Iraq's large outstanding debts and the validity of oil contracts agreed between Saddam Hussein's regime and the three large states which forged a joint stance against the US-led war - Russia, France and Germany.

The noble ambition of 10 years ago, that the EU would heal the scars of the long Cold War division of Europe, is at serious risk

Leaders of those three countries showed more flexibility when they met at the weekend in Saint Petersburg.

In an overture to the Americans, EU foreign ministers this week declared they want a stronger stance against terrorism and the spread of weapons of mass destruction.

But European nations are split over how far to support the United States' uncompromising stance.

The eastern European states which will soon join the Union all say they back America, and want to bind the US to Europe through Nato. But France, with Germany and Belgium, publicly broke the unity of Nato over Iraq.

And later this month, with Luxembourg, they are to launch a new drive to create a so-called defence union within the EU, independently of the Nato alliance.

The Athens agenda has been shifted by Iraq

To add to this rich mixture, the president of the EU's constitutional convention, Valery Giscard d'Estaing, is here in Athens to outline ambitious plans for a new federal-style political structure for the whole Union, including a figurehead EU president and an EU foreign minister.

Mr Giscard's convention, made up of political representatives from 28 countries, is meant to pave the way for another treaty and an EU constitution next year.

But fierce arguments lie ahead.

On Thursday Russian Foreign Minister Igor Ivanov is due to take part in a so-called European conference, a meeting of 40 states from eastern and western Europe which serves as an open forum but has no firm agenda.