`What I would like to say to Milosevic is, `Don't play around with Montenegro'. He should not interfere with Montenegro. I would not like to go any further. He knows that."

The veiled threat is there. Javier Solana does not need to go any further. The EU's congenial High Representative for Common Foreign and Security Policy may have left NATO but is still someone Serbia's leader, Slobodan Milosevic, must take account of.

As head of the developing EU military dimension and of the Western European Union, he is responsible for ensuring that if there is another Bosnia or Kosovo, Europe is capable of responding independently.

It is perhaps a surprising role for a man who less than 20 years ago campaigned against Spanish membership of NATO. Born in Madrid in 1942 to a well-off family, Solana trained as a physicist at the university in the city. It was also the start of his political life in opposition to Franco's stifling rule, in the underground world of the youth section of the Spanish Socialist Workers Party (PSOE).

His student days, interrupted by suspensions for political activity, were spent partly at home and partly in Britain and then in the US as a Fulbright scholar.

With the return of democracy he threw himself into open politics, winning a parliamentary seat in 1977. A confidant of the party leader, Felipe Gonzales, his ministerial career began in 1982. Marxist language gave way to the politics of mainstream social democracy but even in the early 1980s Solana was strongly opposed to Spain's successful bid to join NATO.

By 1986, when the Socialists held a referendum, however, he was a strong advocate of membership and has never looked back, declaring that "only idiots never change their minds".

But NATO was changing too, he would insist as its secretary general last year: "NATO was born when Europe was divided and it has now become a leading instrument in the reconstruction of the continent. This is an incredibly dynamic process. If this pace continues it is hard to predict what NATO will be like in three years' time."

By the time he was appointed to lead the organisation in 1995 he had proved himself to the EU states and the US as the staunchest of converts. He went on to become the chief architect of the Partnership for Peace process of engagement with central and eastern Europe, and a key figure in reassuring the Russians that NATO's expansion should not be seen as threatening. He would run two major military operations, in Bosnian and Kosovo.

Solana's transition to head the EU's new diplomatic and security wing has been seamless. The philosophy of international security - even Ireland's - is all about how the various international security organisations complement one another. NATO and the EU may not be one, but they march in step.

But does he understand the fears of those who say the EU is moving too fast?

"We have decided to give ourselves a military capability for crisis management and this is not to make war but to make peace," he insists. "This is not to conquer territory but to defend others who may be suffering in crises in which a peacekeeping operation may be needed.

"But I must underline this is not a standing army . . . This is not for the collective defence of the EU countries. This is for peacekeeping or peace-enforcing operations under the mandate of the UN or the Organisation for Security and Co-operation in Europe."

Although EU declarations do not rule out acting without a UN mandate, he insists "the 15 members of the European Union are the most important supporters of the UN. It is impossible to imagine that these countries are going to act contrary to the spirit of the UN."

He acknowledges that the ground rules are shifting, particularly in the wake of the tragic experience of Srebrenica in Bosnia. The right of intervention in defence of human rights is becoming more firmly established. "I think the philosophy is beginning to change. And the philosophy basically, as Kofi Anan puts it, is that the Charter of the UN talks about human beings . . . and therefore the rights of those human beings have to be defended. It is that type of debate which is making some headway."

Those who are NATO members "have found the security guarantee there", he says. The limits of EU military engagement are clear and unambiguous.

Is there not already a de facto commitment to protect each other if attacked, as the President of the Commission, Romano Prodi, suggested recently in Latvia? No, he insists. To be of real value security guarantees "have to be de jure", based on treaties, automatic. But anyone who attacked the EU would certainly face political and economic consequences, he argues.

"We don't know yet," he says of the cost. It will be up to each member state to work out how much it can contribute to the headline goal of having the ability to deploy a 60,000-strong rapid reaction force.

Of the Balkans, to which much of his speech to the Institute for European Affairs (IEA) today will be addressed, he cautions that Europe must be committed for the long run.

The co-ordination of the EU's policy in the region, which he shares with the Commissioner for External Relations, Chris Patten, is based on targeted sanctions against the Milosevic entourage and visible rewards for democratisation.

"Serbia is a black hole in the region," he says. "But look at the region - you have two countries, Romania and Bulgaria, which are already candidates for the European Union, you have Macedonia which is evolving in a very stable manner. Croatia too is evolving."

Solana, who knows Ireland well, is on his first visit in his new role. He will be meeting both the Taoiseach, Mr Ahern, and the Minister for Foreign Affairs, Mr Cowen, as well as speaking to the IEA.