Twenty years ago, ten Eastern European countries, amongst them the Baltic states, but also Poland and Hungary, joined the European Union as part of the enlargement to the East. In the past, you have characterised the 2004 enlargement round, which you prepared, as the most significant historical achievement of the EU since its foundation. Do you still share this view today?

Yes, absolutely. Even more so. From today's perspective, it is a minor miracle that it was possible to realise this project. The EU would no longer be in a position to do this today. It was only possible back then because the spirit of a new, united Europe was still alive and governments, parliaments and society in the candidate countries were prepared to take on the hardships of transformation. This was the only way it could be successful. Now, 20 years later, I would say that the main points that needed to be achieved have been achieved. But of course that is not the end of the story. To paraphrase Willy Brandt, not everything that belongs together has grown together yet.

But not all of the countries that joined back then are making positive headlines. With regard to countries such as Hungary, could it not be said that joining the EU was perhaps a little too early?

The same could be said for some old members. Which country always gets the best press? It's interesting that you choose the example of Hungary. Hungary was everybody's darling back in 2000. Hungary was the country that had opened the Iron Curtain. At the time, there was almost a love affair between the EU and Hungary. Nobody would have thought of saying that Hungary did not belong in the European Union. Democratic elections do not always necessarily produce the results that others want. I also don't believe in punishing people for their voting behaviour.

At the moment, nobody would describe the relationship between the EU and Hungary as a love affair.

No, absolutely not. And it hasn't been for ten years. But we shouldn't make the mistake of writing off Hungarian democracy. Hungary is still a democracy and the fate that befell Jarosław Kaczynski in Poland will also befall Viktor Orbán.

In the past, you have also spoken out in favour of Turkey joining the EU and said that there were no more obstacles on the table regarding the country's accession to the EU. How do you assess the situation today?

The EU has lost Turkey. And that is one of the most serious strategic mistakes it has made in recent decades. In contrast to the enlargement to the East, where an act of historical justice took centre stage, the case of Turkey is a geopolitical one. The firm integration of Turkey into EU structures is a central security policy issue, as Turkey is located at the interface between Europe and Asia.

When the European Council recognised Turkey as a candidate country in 1999, the country developed an incredible momentum for reform, which was at its strongest in 2002 when Erdoğan came to power. Today, of course, that has been forgotten. In 2005, there was a change of government in Germany and the referendum on the European Constitution failed in France. Chirac blamed the EU's Turkey policy, among other things, for this. Merkel, in turn, claimed that Turkey was a different culture with a different religion that did not belong to Europe. That was the end of the matter. So why should Turkey bother any more?

Shouldn't this narrative also include a clear criticism of the Erdoğan regime?

No question about it. I am not saying that the EU is to blame for what has happened in Turkish domestic and foreign policy since then. But I am saying that the EU has recklessly relinquished any possibility of influencing Turkey's internal development and Turkish politics. The Erdoğan regime is now coming to an end, but I very much doubt whether we can return to the situation of 1999 or 2004. In that part of the Turkish population for whom it was a heartfelt wish to be accepted as Europeans, the disappointment and frustration is naturally very great.

Let's look towards the Balkans. Several countries, including Albania, Montenegro and Serbia, are on the list of candidate countries. How do you view the forthcoming Balkan enlargement of the EU?

The so-called Western Balkan states have had the prospect of accession since 1999. It should be an incentive for the countries in the region to ensure peace, stability and harmony with their neighbours. This was initially pursued very actively, but after some time the EU did not do much to keep this process alive. One could argue that if the EU had been a little more energetic - as it was with the enlargement to the East in 2004 - we would be further along today. But something else is crucial: this next round will not be possible without changes to the EU's institutional system. The admission of several small and micro states would change the democratic balance within the EU. Under our current system, these states could then determine the course of the EU. There is a broad consensus that the next enlargement must be linked to institutional reform. To give just one example: can you imagine a Commission with 37 members or more, seven of whom would come from former Yugoslavia, but only one from France? I doubt whether such a system would be accepted by the population. Before this accession round is finalised, a new European treaty is needed, which, in view of the right-wing populist wave and the current situation in Europe, will not happen in the near future. The aim to enlarge is still right, but first the question of the EU's continued ability to function must be clarified.

At the end of last year, the EU states decided to start accession negotiations with Ukraine. Realistically, the country is still a long way from becoming a member. Is the prospect of membership for Kyiv an incentive for reforms or purely a symbolic policy that also raises false hopes?

I think both. But I am more inclined to believe that it is a purely symbolic act, which may well have its value for Ukraine in the current situation. However, it does not initially offer Ukraine any realistic prospects of accession. Nevertheless, I do not want to rule out the possibility that a whole range of players in the EU are also in good faith that this prospect can encourage the country to make the necessary reforms. In fact, we are dealing with profound structural problems in Ukraine - endemic corruption, the deep-rooted power of the oligarchs and the radicalism of the ultra-right - which make this process very difficult. Then there is the martial law. In view of all this, I am rather pessimistic about the prospects.

We also do not know what political and territorial form Ukraine will take, and what serious negotiations could then be conducted with it. The negotiations will raise the most difficult issues that are already causing massive anger in some EU states, such as agriculture. And then there are the costs that Ukraine's accession would entail. The funds for this must be raised and distributed. There are only two options here: Significantly increase the member states' contributions or significantly reduce the benefits. Both will be very painful. Above all, they will hit the largest net beneficiary, Poland, so the enthusiasm there for a quick deal will cool down relatively fast when we get close to serious negotiations with Ukraine.

Has the EU made mistakes concerning Ukraine?

Without a doubt. For almost 25 years, Brussels' policy has been to keep Ukraine at arm's length. Even in the Association Agreement approved in 2014, Ukraine's wish to be given the prospect of joining the EU was not fulfilled. In the 2011 negotiations, the EU had simply refused to refer to the EU Treaty, which states that anyone can apply for accession. Until 2014, the European position was clearly characterised by distance. This then changed, mainly due to American and British pressure - and of course the war. But if the EU had had a pan-European strategy, we could have been much further along by 2014. Perhaps even then there would be no war today.

In addition to Ukraine, accession negotiations are also underway with the Republic of Moldova. Georgia is also an official candidate for accession. Isn't there a danger of imperial overstretch for Brussels?

Well, the term ‘overstretch’ is usually used in relation to empires. You can certainly criticise the European Union for a lot of things, but it does not have imperial ambitions. Nobody is forced to be part of it, nobody is oppressed. The idea of European unification is pan-European and the EU is an instrument to create this European unity, because by its very nature the EU is an alliance for peace. The old lesson that Europe is only peaceful where it is united applies undiminished today.

This interview was conducted by Joscha Wendland and Nikolaos Gavalakis.