The Lisbon Treaty takes effect on Tuesday and Catherine Ashton, the EU's new High Representative for Foreign Policy, formally assumes office. Unlike Herman Van Rompuy, whose term as European Council President starts in January, Lady Ashton takes over as soon as the Lisbon Treaty enters into force.

In actual fact, Lady Ashton will only be starting half her new job - besides being the EU's foreign policy chief, she will also be a vice-president of the European Commission. The whole new Commission will have to be approved by the European Parliament, probably in January or February.

One of the highlights of the Lisbon Treaty was to merge the position of European External Affairs Commissioner with that of Foreign Policy High Representative, to have a more coherent EU foreign policy and to avoid overlap, and this certainly makes sense.

Over the past two decades the EU has been trying to work together in the area of foreign policy, and the principle of a Common Foreign and Security Policy (CFSP) was formalised in 1992 in the Maastricht Treaty. This came about more or less during the break-up of Yugoslavia and the civil war in Bosnia - certainly not the EU's finest hour. The EU was powerless to intervene in this conflict and lacked the political will to do so. It was only Nato that eventually forced the Serbs to the negotiating table, which then led to the Dayton Peace accord. The same thing can be said about the conflict in Kosovo a few years later in 1999. It was Nato, not the EU, which stopped the genocide in Kosovo.

So the evolution of the EU's common foreign and security policy has been a slow process, although there are clear examples of success stories. In the aftermath of the Balkan wars and the increasing conflicts in Africa, the EU created a European Security and Defence Policy within the overall framework of the CFSP, where military or police forces can be sent to areas of crisis to carry out humanitarian missions, peacekeeping, crisis management and peacemaking.

Such missions can be found in Bosnia, Kosovo, Macedonia, Georgia, the Congo, and Afghanistan. There are, in fact, thousands of EU peacekeepers around the world.

The EU, of course, does not have a single foreign policy but is attempting to build a common one. Britain will always have a special relationship with the US; Spain will always be particularly close to Latin America; France, Italy and Malta will always give particular importance to the Mediterranean; Finland will always stress the importance of engaging with Russia, while the former eastern bloc EU member states will always be suspicious of Russia. This state of affairs, however, does not threaten EU foreign policy and the EU's diversity can be a plus here.

Foreign policy decisions still require unanimity by the member states, and this is understandable, at least for the foreseeable future. However, a lot of progress has certainly been made and the CFSP has definitely changed the way the EU operates. Is it possible, for example, to imagine a world where the EU had no unified positions on Iran, the Balkans, the Palestinian-Israeli conflict, Africa, Afghanistan or climate change?

Certainly, there are areas of foreign policy where the EU still has no real common positions, such as how to deal with China and Russia. Such a situation only benefits Beijing and Moscow, which takes advantage of a divided EU.

Of course, the war in Iraq was the classic example of how divided the EU can be over a crucial foreign policy issue, something that poisoned the atmosphere in Brussels for many years after the war. Hopefully, the EU will never again be so split as it was over Iraq.

Lady Ashton will now have to use all her skills to persuade member states to adopt common positions on foreign policy as often as possible. According to the Lisbon Treaty, she will "conduct the Union's common foreign and security policy" - but only where common positions exist.

Over the next few months she will have to prove that she is capable of appearing on the world stage with leaders from the US, China and India. She has already said she will pursue a policy of "quiet diplomacy" in her new role, suggesting she would not rock the boat of international diplomacy.

The reaction so far by the international media as well as various analysts over Lady Ashton's appointment has been one of scepticism, considering her lack of experience in foreign affairs. Should another crisis erupt in Georgia, for example, would she be able to pick up the phone and call the Russian President, and would she carry any clout?

Janis Emmanouilidis, senior policy analyst at the European Policy Centre, said: "The fact that Ashton has no foreign policy experience is surprising, because if you look at the challenges ahead, it is rather a negative."

Lady Ashton has a crucial role ahead of her and she will now have to prove she is up to the job. Hopefully, she will rise to the occasion.