There won’t be a horse-drawn carriage ride or a lavish banquet at Windsor Castle, as there was during last week’s state visit of President Emmanuel Macron of France. But when Chancellor Friedrich Merz of Germany visits London on Thursday, it will add another layer to the portrait of Europe uniting against foreign threats.

Britain and Germany are expected to sign an Anglo-German treaty on defense, energy, economic cooperation, and migration, officials from both countries said this week. The defense accord will build on an agreement signed last October, under which the two agreed to cooperate on mutual defense, with joint military exercises and the development of sophisticated weapons.

Mr. Merz, a center-right leader who came to power in May, has quickly emerged as a linchpin in Europe’s effort to build a more independent role in its security since the return of President Trump to the White House. Prime Minister Keir Starmer, who will meet Mr. Merz at 10 Downing Street, has likewise tried to position Britain as a key player in European support for Ukraine in its war against Russia.

Details of the treaty were still being worked out this week, officials said, but it is likely to include a pledge by both countries to regard a threat against one as a threat against the other.

Such a commitment to mutual defense would echo language adopted by Britain and France, which pledged last week to more closely coordinate their nuclear arsenals in responding to threats against European allies.

Germany does not possess nuclear weapons, but it is the third-largest supplier of military hardware to Ukraine, after the United States and Britain, according to the Kiel Institute for the World Economy. Under Mr. Merz, Germany has agreed to increase its military spending to 3.5 percent of gross domestic product by 2029, its most ambitious rearmament since the end of the Cold War.