The leaders of Britain, France and Germany are burying lingering grievances. They are creating new defense partnerships. And, together, they are keeping a wary eye on their longtime ally, the United States.

In the six months that President Trump has rattled the decades-old trans-Atlantic alliance, his counterparts in Europe’s most powerful countries are building parallel diplomatic and defense institutions for a future without the United States as the primary guarantor of economic and military security.

On Thursday, Prime Minister Keir Starmer of Britain and Chancellor Friedrich Merz of Germany signed a wide-ranging treaty for mutual defense, economic cooperation and other partnerships. Last week, Mr. Starmer and President Emmanuel Macron of France agreed to coordinate their nuclear arsenals. In May, the three men traveled together by train to Ukraine for a demonstration of solidarity. Next week, Mr. Macron will visit Mr. Merz in Berlin.

The three men are also leaders of a “coalition of the willing” aimed at supporting Ukraine’s fight against Russia as American support wanes, an effort that will soon get a formal headquarters in Paris. Planning for a possible Europe-led peacekeeping force in Ukraine has been underway for months. On Friday, the European Union announced an 18th package of sanctions against Russia.

The “triangle alliance,” as Britain, France and Germany are sometimes now called, are already partners through NATO and the Group of 7 — forums that include the United States. Officials for the three European countries are careful to say that the institutions they are building are meant to supplement those alliances not replace them.

But NATO is a sprawling defense bureaucracy that represents 32 countries, some of whom disagree with each other. Officials in Berlin, London and Paris are eager for a smaller, more nimble group to respond to what Mr. Merz on Thursday said was a shift in the relationship between Europe and the United States.