With a proposal for a new type of European collaboration that extends beyond the EU, France’s President Emmanuel Macron has revived a decades-old notion, but the specifics of how such a system might function remain obscure and controversial.

A bigger European club might give an option for cooperation on political and security matters for the ever-growing list of potential member states, including post-Brexit Britain, while they await admission, which can frequently take years.

However, Macron’s plan, which he introduced in a keynote speech to the EU Parliament on Monday, has already sparked controversy, not least because it might be offered to Ukraine in lieu of full membership.