With 23 European Union countries launching a programme of joint military investment and project development, is Europe moving closer to creating an EU army?
And, if so, how would it affect Britain?
EU foreign policy chief Federica Mogherini describes the step as a "historic moment in European defence".
"23 member states engaging both on capabilities and on operational steps is something big.”
Their pledge is a sign of political will but the programme will only enter force once it has been legally endorsed, probably in December.
How Could It Lead To An EU ARMY?
Britain has historically pushed back against deepening EU defence commitments, but with it leaving in 2019 it will no longer be able to voice its concern and the EU will find it easier to take these steps.
German Foreign Minister Sigmar Gabriel says the agreement is "a great step towards self-sufficiency and strengthening the European Union's security and defence policy - really a milestone in European development".
For now, the pledge centres around finance, allowing nations to combine funding and spending weight so that expensive capabilities like air transport or drones can be purchased.
"The real problem is not how much we spend,” Ms Mogherini says, “it is the fact that we spend in a fragmented manner.”
Sovereign nations relying on each other to purchase shared military assets will sound to many like a step towards an EU army.
But whilst it does sound like a step – even if only a small one – Ms Mogherini says the move would complement NATO's security aims, not compete against or replace it.
NATO Secretary General, Jens Stoltenberg, says that he welcomes EU leaders’ assertion that – for now at least – “European defence has to be developed in a way which is not competing with NATO but is complimentary”.
The permanent structured co-operation, or Pesco, does not involve Britain, but Prime Minister Theresa May says the government will maintain its commitment to protecting Europe after Brexit.
And there is no knowing how entangled that commitment could become.
May used a speech at the Guildhall in London’s financial district to reiterate Britain's "unconditional" pledge to maintaining European security, including a proposed post-Brexit pact with the EU.
“The comprehensive new economic partnership we seek will underpin our shared commitment to open economies and free societies in the face of those who seek to undermine them.”
What About Existing Joint Defence Projects?
The UK currently has the biggest defence budget of any EU member, making it an important asset to the EU.
Britain has a history of collaborative defence projects with EU countries including the Eurofighter Typhoon aircraft and the Meteor air-to-air missile.
EU foreign policy chief Federica Mogherini says that countries have already submitted more than 50 joint projects in the fields of defence capabilities and military operations.
But Britain is only allowed to take part in these if they are of benefit to the entire EU.
By offering to place its defence programme at the disposal of the EU, the Government hopes to win concessions in trading and economic relations following Brexit.
It means that, at this moment, it is unclear how reliant on EU defence relations Britain may become.
A fact that may be troubling to some given the strengthening of EU defence partnerships after Britain’s departure.