France and Germany have signed a new treaty on Tuesday aimed at breathing new life into their place at the centre of the European Union.
As the UK moves to leave the EU and a rising tide of populism challenges the core liberal values of the bloc, the new treaty commits wholeheartedly to defending it.
German Chancellor Angela Merkel highlighted peace and security and backed the emergence of a European army.
French President Emmanuel Macron said the challenge was for Europe to become "a shield" against the tumults of the world.
There is rich symbolism in the signing in the German city of Aachen, which has changed hands over the centuries and is known in French as Aix-la-Chapelle.
But will it ultimately change anything?
What's in the treaty?
France and Germany agree to establish common positions and issue joint statements on major EU issues - formalising their existing co-operation. They also plan to act as a joint force at the United Nations.
From foreign policy to internal and external security, the two nations commit to coming up with common positions while seeking to bolster "Europe's capacity to act autonomously".
The two countries commit to:
- Deepening economic integration with a Franco-German "economic zone"
- Developing Europe's military capabilities, investing together to "fill gaps in capacity, thereby reinforcing" the EU and Nato
- The possibility of joint military deployments as well as a Franco-German defence and security council
For young people, there is agreement to focus on cultural exchanges and increase learning of each other's languages, with the aim of a Franco-German university.
There are also plans for closer cross-border links and greater "bilingualism" on both sides of the borders.
What did the two leaders say?
German Chancellor Angela Merkel said the treaty came amid "special times" with the rise of populism and nationalism.
"For the first time, a country is leaving the European Union - in the form of Great Britain," she added.
Those who forget the value of peace and spread lies are accomplices in the crimes of the past," Mr Macron said.
"I would rather look our Europe in the face and strengthen it to protect our peoples. That is what we are doing," he added.
Mrs Merkel also said Germany wants to "make our contribution to the emergence of a European army."
"We are committed to developing a common military culture, a common defence industry and a common line on arms exports," Mrs Merkel said.
The idea is not new - both leaders have called for a common European defence force that would operate within - and not replace - Nato.
Nato's Secretary General, Jens Stoltenberg, gave the treaty his approval, saying Nato had been briefed on the military matters.
"For decades, Franco-German cooperation has been essential for security and stability in Europe," he said, adding that the treaty was a reminder "of how far Europe has come since the devastation of the Second World War".
How ambitious is it?
Many of these aspirations have been heard before.
Exactly 56 years ago, the first Joint Declaration of Franco-German friendship was signed in Paris.
"Since then, the spirit of the 1963 treaty has been evoked time and again by different French and German governments," says Dirk Leuffen, chair of Political Science and International Relations at the University of Konstanz.
Prof Leuffen believes there is no dramatic shift but instead "it continues or translates the old goals into today's challenges".
He singles out the economic plans as potentially signalling a next step of aligning the two countries more closely.
For Alistair Cole of Cardiff University, the importance of the treaty lies in its symbolism. In the context of a post-Brexit Europe he believes it is intended to "declare the centrality of France and Germany, though in practice the two countries often do not see eye to eye".
Is there much opposition to the pact?
European Council president Donald Tusk - a native of Poland - sounded a note of caution in a speech he gave at the ceremony.
"Germany and France can, and should, serve the whole of Europe well," he said.
But he added: "I will put it bluntly - today Europe needs a clear signal from Paris and from Berlin, that strengthened co-operation in small formats is not an alternative to the co-operation of all of Europe. That it is for integration, and not instead of integration."
Central and Eastern European states have refused to accept German and French leadership on migration.
"It is time to oppose the Franco-German axis with an Italian-Polish axis," said Italy's right-wing Interior Minister Matteo Salvini on the day the final treaty draft was announced.
He was speaking on a visit to Poland, aiming to challenge France and Germany's dominance in the EU with a Eurosceptic alliance ahead of May elections to the European Parliament.
The treaty itself has been the subject of considerable fake news in France, with conspiracy theories about Mr Macron going to "sign over" territory as part of the deal.
One French MEP, former National Front member Bernard Monot, claimed in a video that the treaty would effectively cede the Alsace and Lorraine border regions to Germany.
The allegation spread quickly on social media, despite several debunking pieces in the mainstream news media.
Far-right leader Marine Le Pen tweeted on Tuesday that the extent of the links with Germany amounted to "treason" and "serious abandonment of our sovereignty."
Why Macron and Merkel care
By Jenny Hill, BBC News, Berlin
Behind the beautiful old stone edifice of Aachen's historic town hall, two allies warmly pledged to deepen their relationship.
But outside, in the bitter cold, Eurosceptic protestors and yellow vest demonstrators shouted at EU supporters.
The treaty - critics argue - is short on substance and risks alienating other EU member states. And as Mrs Merkel and Emmanuel Macron emerged to greet the crowd outside, their smiles looked a little weak.
Aachen is about ambition, vision and symbolism. But Mrs Merkel's power is waning and Mr Macron is struggling at home. Sceptics wonder what will become of their promises once their time in office is done.
Will the treaty change anything?
President Macron's power has waned since he was elected in 2017 with a commitment to a series of pro-European measures, including a common budget for the eurozone.
Prof Cole believes the main issue is not the treaty but the "very uncertain future of Macron's European reform programme".
However, he believes there may be some movement towards a more integrated security and defence strategy and, more controversially, towards debt-sharing in the eurozone.
In the words of German Foreign Minister Heiko Maas, the two countries "are joining forces to fight for a strong Europe that is capable of taking action, a peaceful world and a rules-based international order".