German Chancellor Angela Merkel gives a press conference during a European Council meeting, on June 22, 2017, at the European Union headquarters in Brussels | John Thys/AFP via Getty Images EU leaders launch ‘historic’ plan for joint military force Agreement on further military cooperation was ‘ambitious and inclusive,’ says Council President Donald Tusk.

EU leaders meeting in Brussels agreed on a plan for enhanced defense cooperation that could pave the way for a joint military force operating under an EU flag.

Pushed by Berlin, Paris, Rome and Madrid, the plan activates a previously dormant mechanism in the 2009 Lisbon treaty that allows an opt-in approach, with countries permitted to launch joint security projects without requiring all EU members to agree. Such was the degree of support at the European Council meeting that the plan was agreed with just "five minutes" of discussion, a senior diplomat in the room told POLITICO.

Following the agreement, German Chancellor Angela Merkel told reporters, "This is an example of an area where we have moved very fast in the last couple of months and achieved significant progress."

European Council President Donald Tusk called the move a "historic step" that would allow the EU to move toward deeper defense integration. "Our aim is for it to be ambitious and inclusive, so every EU country is invited to join. Within three months, member states will agree a common list of criteria and commitments, together with concrete capability projects, in order to take this cooperation off the ground."

The defense cooperation plan is an example of EU countries pursing greater integration with the U.K. heading for the exit door. The British government has previously thrown up obstacles to greater military cooperation on the grounds that it risked undermining NATO through duplication of effort and capability. But with Brexit negotiations underway and NATO and the EU cooperating more closely in recent years, those objections have melted away.

Merkel said the plan would not undermine NATO. "Everyone stressed today that an orderly common set-up of defense structures should not occur in opposition to NATO but the contrary — in collaboration with it. However, within the EU there is the possibility to integrate much more those member states that are not members of NATO."

The president of the European Parliament, Antonio Tajani, argued that cooperation with the U.K. was still essential. “It's important to keep collaboration with the U.K. in fields such as terrorism and defense. Let's not forget the U.K. remains a NATO member,” he said.

Though there was broad agreement among leaders, there is tension between Germany, which wants to make it as inclusive as possible so that any member country can take part, and France, which is pushing for a more ambitious but also more streamlined approach. There are also differences between those such as German Defense Minister Ursula von der Leyen, who views the plan mainly as a chance to develop military capability, and those who think that it could have an operational role in the field. Ambition means “not only to be able to build capabilities but also to perform missions,” said a senior EU official.

The details will be thrashed out at future European Council summits. “I expect strong leadership from our heads of state and government to give further impulse so that foreign and defence ministers together with me can follow up in the coming months and come back in December with even more decisions taken,” Federica Mogherini, the EU's foreign policy chief, told reporters before the summit started.

The agreement follows a pledge by the European Commission to create a €1.5 billion-a-year fund from 2020 to boost European defense capability.