By midday Saturday in Europe, a question was bouncing between the government offices and vacation villas of the continent’s most influential leaders. The Ukrainian president was headed to the White House for a crucial meeting with President Trump. Mr. Trump was allowing him to bring backup. But who should go?

It was the sort of dilemma that once might have erupted into public disputes between Germany, France and Britain, the continent’s largest powers. This time, it didn’t.

The leaders of those countries decided they would all accompany Volodymyr Zelensky, the president of Ukraine, to Washington, for a summit with Mr. Trump about peace talks with Russia. So would the leaders of Italy, Finland, the European Union and NATO.

They flew in on separate planes. But with Mr. Trump, they spoke in one voice.

“We were well prepared and well coordinated,” Friedrich Merz, the chancellor of Germany, told reporters after he and his counterparts met Mr. Trump at the White House. “We also represented the same viewpoints. I think that really pleased the American president.”

Mr. Trump’s persistent and sometimes volatile effort to bring a diplomatic end to the war between Ukraine and Russia has forged stronger bonds among European leaders. It has strengthened the unity that emerged earlier this year amid Mr. Trump’s tariff threats and his wavering on what have been decades-long security guarantees that America has provided to Europe.

Since Mr. Trump’s election, European leaders have raced to shore up their own defenses, wary of losing American support. NATO members, led by Germany, have pledged to increase their military spending significantly, to meet a target set by Mr. Trump.