WHEN it was originally signed in 1963, in the long wake of World War II and with the Cold War deepening, the Elysee Treaty served to reconcile Germany and France and establish their relationship as “an indispensable stage on the way to a united Europe.”

On Tuesday, Chancellor Angela Merkel of Germany and President Emmanuel Macron of France met in a German city symbolic to both — Aachen, or Aix-la-Chapelle in French — to renew that commitment for the 21st century, in a ceremony that nevertheless served as a reminder of the daunting array of challenges threatening Europe today.

The leaders and their countries, former enemies who lost millions in wars last century, form the staunchly pro-Europe core of the Continent. But with Merkel already on a glide path out of power and Macron severely weakened by popular protests at home, their simultaneous decline is threatening to leave a crater at the center of Europe’s decades-old project of unity.

Internal and external forces continue to raise the prospect of a fracturing of the European Union. Britain is scheduled to leave the bloc March 29. The Trump administration is threatening tariffs and questioning Washington’s commitment to NATO. Populist governments in Hungary, Italy and Poland are challenging fundamental principles of liberal democracy and the rule of law.

“Ms. Merkel and Mr. Macron are sending a strong signal: At a time when nationalism is gaining more and more ground in Europe, the two are renewing the vow for cooperation and unity,” said Ronja Kempin, a senior fellow at the German Institute for International and Security Affairs in Berlin. “Even though they cannot stop Brexit and ban populism, they are showing what Europe is supposed to look like and how it is supposed to work.”

The French-German friendship has served as the backbone to the European Union as it grew from a predominately economic alliance of West European countries to a political bloc of 28 stretching across Europe. Yet many of the newest members, particularly in the East, look warily at the strength of the friendship between Paris and Berlin.

While the “Franco-German couple,” as the relationship is commonly called in France, remains vital to the continent’s future, critics say the new treaty is a relatively weak renewal of vows. It also risked angering many of the two countries’ smaller European partners, who increasingly bridle against what they see as the imposition of German and French priorities, especially when it comes to budgets and migration.

As if further reminders were needed, a diplomatic spat this week between Italy and France proved the point. Over the weekend, a deputy prime minister of Italy, Luigi Di Maio of the populist Five Star Movement, called on the European Union to impose sanctions on France for its policies in Africa, “which are impoverishing these places” and encouraging migration.

(SD-Agencies)