With the recent signing of the Treaty of Aachen, French President Emmanuel Macron and German Chancellor Angela Merkel have renewed the Franco-German friendship pact and taken an important and necessary step forward for Europe. But the United Kingdom should not have been left out.

The U.K. is an integral part of Europe; as the European Union's second-largest economy, its GDP equals that of the 19 smallest EU member states combined. Its exodus thus would shake Europe to its core and destroy the European postwar order.

Moreover, it is worth remembering that in 1963, the Bundestag prefaced the Elysee Treaty with a preamble stipulating that Germany hoped to bring Britain into the European Economic Community; in 1973, that is what happened. A similar overture to Britain would be no less appropriate today.