Ms. Merkel herself has been a source of disappointment for Mr. Macron. Her natural caution has been cold water to his impassioned plans for European reform, especially for the countries that use the common euro currency. Most of his lofty visions, like a shared eurozone budget, have been watered down or have come to nothing. But Ms. Merkel also wants to support him and his commitment to Europe, especially when he is in difficulty at home.

Each leader acknowledged the challenges, but drew on them as reasons for the new treaty.

“Seventy-four years after the end of World War II — a lifetime — things considered self-evident are again being called into question,” Ms. Merkel said in remarks before the signing. “That is why we need, first, a re-establishment of the responsibility of Germany and France within the European Union and, second, a reorientation of our cooperation.”

The new accord includes pledges to deepen economic integration through a joint “economic zone,” and to reinforce cooperation in military purchasing and coordination with the aim of improving Europe’s ability to act on its own. It includes a mutual defense pact, within the context of NATO membership, and would establish a joint defense and security council.

Both Mr. Macron and Ms. Merkel have called in their own ways for a European army capable of acting alone when NATO does not wish to be involved. Both have also said that Europe can no longer count on the United States, as it has in the past. That idea created anger and concern in Washington and set off a series of contemptuous remarks over Twitter from President Trump.

“Europe is the protective shield for our people against the new storms in the world,” Mr. Macron said.