Prospect of Trump’s return to dominate European Security Summit

Trump’s pick of Sen. JD Vance (R., Ohio) as his running mate on Monday reinforced a sense in Europe that the former president’s return to office could mean a dramatic drop in U.S. aid for Ukraine and a push to force Kyiv into peace talks with the Kremlin. It could also mean a U.S. pivot in defense priorities toward Asia to deter China that would leave Europe to increasingly fend for itself.

At a meeting Thursday, European leaders from across the Continent will confront the challenges facing the region if the U.S. pulls away, including whether Europe can muster the unity and military muscle to check Russian aggression and keep Ukraine afloat.

The U.K.’s new prime minister, Keir Starmer, will host 45 leaders from the European Union and nearby countries such as Turkey and Ukraine, the fourth such meeting of the so-called European Political Community, an initiative driven by French President Emmanuel Macron following Russia’s invasion of Ukraine in 2022. The idea was to give the region beyond the 27-member EU a chance to talk over security issues and threats independent of allies such as the U.S. and potential foes such as Russia and China.

Officially, the summit will focus on the issues like tackling migration and efforts to build democratic resilience against Russian hybrid attacks such as disinformation campaigns as well as the Ukraine war. British and EU officials also view the gathering at Winston Churchill’s birthplace—the 18th-century Blenheim Palace near Oxford—as a chance for Britain’s new government to reset relations with the EU, which Britain left in 2020.

Yet leaders are set to discuss on the sidelines the possibility of Trump’s return and the security challenges that will pose, officials say.

Trump, like previous presidents, has criticized Europe for not pulling its weight in military spending and relying on U.S. power as part of the North Atlantic Treaty Organization, which requires the U.S. and other members to defend any NATO state from attack. Since Russia’s annexation of Crimea in 2014, most European nations have started to slowly increase military spending but still largely rely on U.S. protection.

Trump has questioned the need for NATO and said he wouldn’t defend countries in the organization that didn’t meet the minimum military spending guidelines of 2% of their own annual economic output. He has also repeatedly questioned U.S. support for Ukraine and promised to end the war, without offering specifics.

The concerns go beyond security. Trump, in an interview with Bloomberg News published Tuesday, repeated threats to hit the EU with new trade tariffs, saying European countries “treat us violently."

Two contrasting responses to Trump have emerged in Europe, both seeking to ensure some independence of action.

On one side stands Hungarian Prime Minister Viktor Orban. Orban’s pitch, laid out in a recent letter to a top EU official, is that the bloc should shift its Ukraine strategy away from war and work with Trump to end the war. That approach would place the EU at the heart of diplomatic efforts to reshape the security order in Europe and avoid costly efforts to support Ukraine alone if the U.S. pulls back, he argued.

“We have not had a sovereign or independent European strategy," Orban wrote to European Council President Charles Michel.

For most European leaders, however, Orban’s approach would sell out Ukraine and leave Europe exposed to Putin’s military ambitions.

Supporters of Ukraine believe that Europe’s best option is to intensify efforts to expand military spending and Europe’s defense industry. That would weaken Trump’s argument that Europe was free-riding on U.S. taxpayers. It would also allow Europe to continue helping Ukraine and build the continent’s deterrence to Russia.

Last month, NATO said that 23 of the alliance’s 32 members were now meeting the 2% target. Last year only 10 members did. Military spending by NATO members excluding the U.S. will reach $430 billion this year in 2015 prices, up from $250 billion in 2014, NATO said recently. By contrast, the U.S. spends 3.4% of GDP on defense, two-thirds of the NATO total.

“We are not complacent," said NATO Secretary-General Jens Stoltenberg at the alliance’s summit in Washington last week. He said current European spending is still insufficient and will rise.

Yet there are fundamental doubts about Europe’s ability to increase military spending speedily at levels needed to keep Ukraine in the fight against Russia and rebuild its own military capabilities without relying on large-scale U.S. assistance.

While Europe’s defense companies are increasing production of ammunition and other vital supplies, critical armaments and equipment, including air defenses, are in short supply and lifting those levels will take years, executives and officials say.

Doubts about Europe’s ability to act faster have increased in recent months, as several big countries, including France and Germany, have hit political crises.

In Germany, the political push behind a surge in military spending that Chancellor Olaf Scholz announced after Russia invaded Ukraine in February 2022 is dissipating, according to critics outside the government.

The German government’s 2025 draft budget, unveiled two weeks ago, earmarked an additional €1.2 billion, the equivalent of $1.3 billion, for the Defense Ministry compared with 2024, leaving it around €50 billion, where it has remained for the past two years, lagging inflation.

Hans-Peter Bartels, a former parliamentary commissioner for the military and now president of the German Society for Security Policy, an armed-forces lobby organization, said Scholz’s push is “on standby."

Government officials reject the accusations, saying that broader military spending, including money from a special 2022 post-Ukraine invasion defense fund, would top €80 billion next year, up around 6% from this year and bringing it to 2.19% of Germany’s GDP. Still, they agree the Defense Ministry will need to stretch the timetable of some of its weapons programs.

Ivo Daalder, a former U.S. ambassador to NATO, said that quiet discussions have been happening for a while now among European defense and foreign ministries about how to shore up European deterrence without a large U.S. contribution. What they need is one thing they may no longer have: time.

“They would like to have another four years," he said, referring to the next U.S. presidential term.

Bertrand Benoit contributed to this article.

Write to Laurence Norman at laurence.norman@wsj.com