NATO Secretary General Mark Rutte said Thursday that next month’s NATO summit in Ankara, Turkey, will further demonstrate Europe’s commitment to a stronger military.

Mr. Rutte vowed that the 32 members of the alliance will show the world — especially Russian President Vladimir Putin — that NATO is fully prepared “to defend all our citizens and every inch of our territory.”

“Last year in The Hague, we agreed to invest 5% of GDP in defense by 2035, a historic decision and a daunting, if essential undertaking. The direction of travel is clear, and allies are stepping up,” Mr. Rutte said in an address to the Atlantic Council during a quick visit to the U.S.

Several NATO member states from Eastern Europe, whose countries are in Russia’s shadow, are already meeting or even exceeding the goals set at last year’s NATO summit in the Hague, Netherlands.

Allies in Western Europe, long criticized by the U.S. for allowing their militaries to atrophy since the end of the Cold War, are now taking Russian aggression seriously, he said.

“Germany is on track to double its investment in defense by 2029 from just a few years ago,” Mr. Rutte said. “This will amount to over 150 billion Euros per year. That is magnitude. These countries are leading by example, and it is just the beginning.”

A decade ago, NATO members wouldn’t have been discussing greater defense spending. That changed under intense pressure from President Trump during his first term. His firm stance caused friction with Europe but also produced results.

Mr. Rutte, a former prime minister of the Netherlands, lauded the progress toward bigger defense spending in Europe.

“Allies united around the imperative to invest in our security, committed to fairly sharing the responsibility, strong and getting stronger,” he said.

While Russia’s full-scale invasion of neighboring Ukraine convinced wary European leaders that their countries could be next, Mr. Rutte said Mr. Trump deserves much of the credit for pushing them to increase their defense investments.

“The threats we face are a key factor in this mindset shift, but I am also convinced that we would not have made such an astounding leap without President Trump’s leadership,” he said. “Every U.S. administration since President Eisenhower called on America’s allies to invest more and take burden-sharing seriously, and one president got them to do it.”

He said European allies and Canada are on a trajectory to equal U.S. defense spending and are taking more leadership roles within the NATO alliance.

“These countries are leading by example, and it is just the beginning. In Ankara, allies will put forth concrete plans for increased investment. Those of you who have been following NATO for a while now understand just how profound a change this is,” Mr. Rutte said. “After years and years of underinvestment, we have seen a sea change in the approach to defense spending over the last decade.”

European countries and Canada are now on a path to equal the U.S. in defense spending. Industries on both sides of the Atlantic are adding additional production lines to keep up with the demand, officials said.

“That’s important, but we need a whole lot more,” Mr. Rutte said. “It’s not only about inventory, but it’s also about delivering effects, firing novel solutions to deal more efficiently and effectively with the security threats we all face.”

Lord Hastings Ismay, NATO’s founding secretary general, said the purpose of the alliance was to “Keep the Russians out, Keep the Americans in, and keep the Germans down.”

Mr. Rutte said Russia has been NATO’s long-term threat for more than 77 years. The Kremlin is investing more than 40% of its budget in defense, churning out military equipment around the clock. But the alliance is keeping a wary eye on other parts of the world.

“China continues to modernize its forces and to expand its nuclear capabilities without any transparency, investing massively in military technology and innovation,” he said.

North Korea is persistent in growing its nuclear program while gaining valuable experience from its support of Russian operations in Ukraine. And then there’s Iran, Mr. Rutte said.