The goal of European diplomacy is to show that if a rogue nation such as Iran takes on one of the EU member states, it takes on all of them.

But if this sounds like the "All for one and one for all" motto of the Three Musketeers, think again. There are 27 EU nations, all with their own foreign policy objectives and diplomatic styles.

The EU was extremely quick to issue a strong condemnation of the Iranian authorities for the way they brutally cracked down on demonstrators protesting that last month’s presidential election was rigged.

Partly the rapid response was down to luck — EU foreign ministers just happened to have a scheduled meeting on the Monday after the vote. Partly it was down to the contrast with an even more cautious approach from the Obama Administration after the President’s inaugural speech pledge to "extend a hand" to hostile regimes.

The cracks in EU co-ordination began to show when the stakes were raised by the arrest of nine British embassy staff.

Britain sought a dramatic early response and won a tough statement under the guidance of the Czech presidency of the EU. Then July 1 brought the Swedes into the rotating EU chair and, with them, a more cautious diplomatic style.

British calls for the mass withdrawal of EU ambassadors remain on the table but have become something of a last resort amid nervousness in some capitals that this measure was too aggressive.

It demonstrates yet again the difficulty of moving the EU beyond rhetoric and into co-ordinated action.

Iran is a country that rightly fears tough EU action. In the face of US embargos, the EU is Iran's largest trading partner, accounting for one third of exports worth €11 billion (£9 billion). The two biggest opponents of sanctions against Tehran have been Germany and Italy — the countries that do the most business with Iran. The huge difficulty in uniting 27 different approaches has hamstrung Javier Solana, the EU’s external affairs representative, who was chosen for his ability not to upset the big member states by trying to take radical positions.

But a strengthened EU high representative, a post already dubbed the EU foreign minister, will be created under the Lisbon Treaty, should it pass a second referendum in Ireland this autumn.

The personality of the first incumbent could shape a more dynamic EU presence on the world stage. One of the main candidates is Carl Bildt, the Foreign Affairs Minister of Sweden, who will use the EU presidency as a shop window of his talents.

The Iran crisis is clearly his first test. British diplomats, already wary of Mr Bildt’s preference for rhetoric over action, are not keen on his appointment to the EU stage.

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