The European Union has signed an agreement with Nato to allow the exchange of confidential information between the two organisations.
EU troops will replace Nato peacekeepers in Macedonia
The accord was signed by the Greek foreign minister George Papandreou - whose country holds the rotating EU presidency - and Nato secretary general George Robertson at a meeting of EU defence ministers in Athens.
The agreement, which comes after months of negotiation, means the Europeans will be able to use Nato's logistical and planning facilities.
It will also pave the way for the replacement of Nato peacekeepers by EU troops in Macedonia - in what will be the EU's first ever military operation.
Common foreign policy
The meeting is another step towards the EU goal of creating a common foreign and security policy.
Around 300 European peacekeeping troops are expected to be deployed in Macedonia later this month, which will be a critical moment in realising this goal.
According to the BBC's Richard Galpin in Athens, if enough progress is made in establishing the EU's military wing, it is possible it will also take over the much bigger Nato peacekeeping operation in Bosnia next year.
The Iraq issue has divided Europe
But the concept of a common foreign and security policy has been thrown into disarray by the Iraq crisis.
European member states are deeply divided on how to disarm Saddam Hussein's regime.
Following the signing of the agreement in Athens, the ministers are continuing talks outside the capital at Vouliagmeni, where they are expected to agree on the detail of the Macedonia deployment over the next two days.
The mission is due to be formally approved by EU foreign ministers next week.