AACHEN, Germany — Angela Merkel and Emmanuel Macron did little more than tinker with the Franco-German engine on Tuesday — but even that was enough to stoke fears among other Europeans that they will roar off on their own.

The two leaders signed the Treaty of Aachen, a 16-page contract seeking to boost cooperation in areas such as foreign policy, defense, trade, mobility and research. The ceremony, in the historic coronation hall of the west German city, took place exactly 56 years after the signature of the Elysée treaty, a highly symbolic post-war accord aimed at burying centuries of enmity between the two countries.

"At a moment where our Europe is threatened by the nationalist movements emerging inside its borders, shaken by a painful Brexit and worried by global concerns that go beyond nations — the climate, digital technology, terrorism or migration — Germany and France have to take responsibility and show the way," Macron said before the signing ceremony.

Aachen was not chosen as the venue by chance. The city, known to French speakers as Aix-la-Chapelle, was the capital of the Franconian King Charlemagne, sometimes called the "father of Europe," whose empire stretched over large parts of what later became the European Union's six founding countries, including France and Germany.

The new treaty is more cautious than ambitious — long on aspirations and short on concrete commitments to make major changes. But talk of revving up the engine that has been at the center of the European project makes some other members of the EU nervous: They fear it could exclude them from decision-making as Berlin and Paris become ever more dominant inside the bloc after Brexit.

“A Franco-German axis that goes around other Member States to stitch up EU business is exactly what we feared when the U.K. announced their intention to leave" — Czech MEP Jan Zahradil

European Council President Donald Tusk gave voice to those fears with a candid warning to Macron and Merkel at the ceremony.

"Let me put it very bluntly: Today Europe needs a clear signal from Paris and Berlin that enhanced cooperation in smaller formats is not an alternative to the cooperation of Europe as a whole," he said.

"To the east of Germany there are hundreds of places where the European spirit of a place — genius loci — is felt as strongly as in Aachen, Paris or Berlin, and where millions of people live whose hearts beat for Europe, a Europe of mutually supportive and equal nations," said Tusk, who spoke in Polish just to make sure no one missed his point.

Euroskeptic Czech MEP Jan Zahradil was even more outspoken. “A Franco-German axis that goes around other Member States to stitch up EU business is exactly what we feared when the U.K. announced their intention to leave," he said in an emailed statement.

According to Zahradil, the Aachen treaty demonstrates why the EU should not deviate from the principle of unanimity in Council decisions on foreign policy: "No other member state is going to feel comfortable giving up their veto if it's just for Paris and Berlin to take the reins."

In Aachen, European Commission President Jean-Claude Juncker argued that critics of Franco-German cooperation should be careful what they wish for.

"For many years I have seen that these common ideas, this platform that you two countries share, may well cause irritation with others. They say, 'oh, that's a bit much.' You're kind of too much of a good thing," he said. "But it would be very painful as soon as Germany and France were not of one and the same position. So we should stick to this common position because that means that the others can also forge a position together."

Elysée echoes

Just like the 1963 Elysée accord, the new treaty avoids naming precise commitments and goals in most areas, and in many ways demonstrates how little Macron has won from Berlin on Europe.

The document says that Germany and France will "deepen their cooperation on foreign policy, defense, external and internal security and development issues, while strengthening Europe's capacity to act independently." However, the commitments to close coordination at European and international level are largely a reflection of what's already common practice between Paris and Berlin.

Both countries promise to defend each other in the event of an attack — something they are already committed to as NATO members. They vow to accelerate the consolidation of the defense sector in order to "further develop Europe's military performance, coherence and credibility."

"Those who forget the value of French-German reconciliation are making themselves accomplices of the crimes of the past" — French President Emmanuel Macron

Elsewhere, France commits to lobby to get Germany a permanent seat in the U.N. Security Council and the two countries agree to more regular high-level meetings of the German-French security council, and to "develop a common approach for arms export."

How difficult the latter issue will be was just recently demonstrated by the Franco-German discord over arms exports to Saudi Arabia: Merkel blocked them after the killing of journalist Jamal Khashoggi, Macron did not.

Pressed on that example during a one-hour dialogue with German and French citizens following the treaty signing, Merkel admitted difficulties: "We thought a great deal about that, as that's about the question of how autonomous a country wants to be," she said. However, she insisted on the need for a common position on the development and export of arms: "We are running around the world selling two airplanes, the Eurofighter and the Rafale. That doesn't make sense,” Merkel said, adding: “We can't lock horns over every screw” that gets exported.

A Europe that protects

Probably the most tangible outcomes of the treaty relate to cross-border cooperation, transport and trade: Both sides want to create "a Franco-German economic area" that cuts bureaucratic hurdles and establishes common regulations, laws and taxes for business on both sides of the Rhine.

However, Merkel cautioned this is a long-term project. "Take, for example, business taxes or insolvency law — much is historically structured in very different ways," the chancellor said during the discussion. "We won't change that overnight, it will take two decades."

Macron stressed the importance of such projects to make the benefits of European integration better visible to citizens and to deliver "a Europe that really protects," echoing a slogan that has become his catchphrase.

The French president, who is facing fierce opposition at home from the Yellow Jackets movement — about 100 of them gathered outside the Aachen town hall to barrack him with howling and whistling — also hit back at opponents of the treaty such as National Rally leader Marine Le Pen, who has accused Macron of handing the Alsace and Lorraine border regions back to German tutelage.

"Those who forget the value of French-German reconciliation are making themselves accomplices of the crimes of the past," Macron said. "Those who ... spread lies are hurting the same people they are pretending to defend by seeking to repeat our history."