"It's going to be quite a historic day for European defense", EU foreign policy chief Federica Mogherini told reporters before the meeting in Brussels at which ministers approved the plan.
European Union member states will be able to develop military capabilities, invest in joint projects and increase the readiness of their troops. Denmark and Britain, which have a special opt-out status, are not expected to participate.
Poland has chose to join the Permanent Structured Cooperation (PESCO) in the field of defense within the EU. The agreement is part of efforts led by Germany and France to push closer defence ties as a way of rebooting the bloc after Britain's shock vote to leave and follows the announcement in June of a European Defence Fund that will receive 5.5 billion euros annually.
"Today we will launch a new page for the European Defence", said Frederica Mogherini, the EU's foreign and defence policy representative.
By working together on joint projects, nations hope to use their combined spending power to overcome capability gaps, jointly buying equipment like air transporters or drones.
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The news comes as Veterans for Britain spokesman David Banks warns that Britain is not only on the hook for European Union spending on defence integration, but actually being entangled in various plans for ad hoc joint headquarters, procurement, research schemes itself.
A general view on a plenary hall as 23 EU member states sign the notification on Permanent Structure Cooperation (PESCO) on the margin of a foreign affairs council at the European Council.
Former UKIP leader Diane James MEP has suggested that, "as the EU Common Defence Fund is being launched way before Brexit, the UK will be providing 13 per cent of the cash" - although this is not yet fully confirmed.
Also, warnings by U.S. President Donald Trump that European allies must pay more towards their security seemed to have propelled the project forward. The EU, she said, has tools to fight hybrid warfare - the use of conventional weapons mixed with things like propaganda and cyber-attacks - that the military alliance does not have at its disposal.
Participants have signed up to a list of commitments which "include increasing the share of expenditure allocated to defense research and technology with a view to nearing the 2 percent of total defense spending" and to "regularly increasing defense budgets in real terms".