Trump wants NATO to spend more. Europe pitches redefining defense to get there.

The issue of “increased Allied defense investment" will be a central focus of talks this week when Secretary of State Marco Rubio meets the North Atlantic Treaty Organization’s other foreign ministers at the alliance’s headquarters, the State Department said. Military outlays have been a sore point in trans-Atlantic relations since Trump’s first term.

Trump has told NATO’s other members to lift their defense-spending target from 2% of gross domestic product—their goal since 2014—to 5%. European members of the 32-country alliance last year focused on 3% of GDP as a new target, and many blanched when Trump mentioned the 5% target after winning re-election.

At a meeting of NATO’s governing council last Wednesday, the U.S. prodded other members to agree to 5% of GDP as their new goal, according to people briefed on the conversation. Diplomats say there is already a consensus building among allies to move toward a 3.5% spending target.

Separately, European allies are also discussing broadening what should count under NATO’s definition of military spending, which could come on top of the 3.5%.

NATO has strict rules for what counts as military spending, but they could change if members agree to it. Officials are holding informal discussions on expanding the list, diplomats say. A broader list is justified by changes in warfare and increased threats, proponents say.

However, there is wariness among some officials that they not look as if they were trying to artificially meet Trump’s target by including marginally relevant or existing expenditures.

Europe has long spent much less than the U.S. on defense and Trump has said this was a sign that European countries have taken advantage of the U.S. During Trump’s first term, he said the 2% target should be doubled and threatened to withdraw the U.S. from NATO if allies didn’t step up.

Rubio recently played down the threat of Washington leaving the alliance while reiterating Trump’s position.

“The only thing that puts NATO in jeopardy is the fact that we have NATO allies who barely have militaries or whose militaries are not very capable because they’ve spent 40 years not spending any money on it," Rubio said on Fox News in February. He singled out “rich countries, especially in Western Europe," for underspending.

Defense Secretary Pete Hegseth reiterated Trump’s target at a meeting of NATO defense ministers in February. “Two percent is not enough," Hegseth said. “President Trump has called for 5%, and I agree."

Last year, 23 members of the alliance hit or exceeded 2% of GDP, according to NATO. A few countries located close to Russia, including Poland, Lithuania and Estonia, are already on course to spend much more than 3% of GDP. For most European countries, getting to 5% quickly would be a painful stretch under current budget planning, amid sluggish economies and growing international tensions.

The European Union, which doesn’t invest directly in weapons but is working to support Europe’s defense sector, recently laid out plans that could help member states meet Rutte’s repeated goal of getting military spending “north of 3%." The EU recently created a new 150 billion euro defense fund, the equivalent of $162 billion, and the bloc is changing debt rules to exempt military spending from its national debt limit. If used fully, the changes would take most of the EU’s 23 NATO members close to 3.5% of GDP by 2030.

Rutte’s “north of 3%" figure is based on the needs determined by the alliance’s new war plans. After Russia’s full-scale invasion of Ukraine in 2022, NATO’s supreme allied commander for Europe, U.S. Army Gen. Christopher Cavoli, began drawing up new mobilization scenarios in case of a Russian attack or invasion. They are NATO’s first full-scale new battle plans since the Cold War.

The detailed yet flexible blueprints, which are classified, set out military requirements for each country. The countries, in consultations with Cavoli’s team, are reaching agreements on their commitments of troops and equipment.

Those commitments would require spending that could reach 3.5% of GDP, NATO officials estimate. Getting to 5% within current plans would be a challenge, say the military, in part because training new troops and acquiring weapons takes time. Arms producers face yearslong manufacturing backlogs and more spending now would simply drive up prices rather than boost arsenals, officials say.

Hegseth called for “real investment, real urgency" when he addressed NATO in February. Some European officials say 3% could be a near-term target and 5% a longer-term objective. The U.S. spent roughly 3.4% of GDP on its military last year, according to NATO. Advocates of Trump’s target say Europe must spend 5% to make up for years of underinvestment.

People familiar with the discussions say one option under initial consideration is to include national spending to improve a country’s resilience to attacks on vital infrastructure, or hybrid attacks that aim to undermine public opinion and trust in democratic institutions.

Another possibility is letting countries include investments that are vital to military operations, such as transportation infrastructure that militaries rely on. NATO includes some military infrastructure in its spending total, but not civilian infrastructure that might be used by militaries in wartime.

During the Cold War, European NATO members kept ports and railcars ready to move military forces. Roads, bridges and port facilities were designed with military specifications in mind. Since the Warsaw Pact collapsed, Europe hasn’t prioritized military mobility. After Russia seized the Crimean Peninsula from Ukraine in 2014, NATO leaders have called for greater focus on the vital enabling capabilities.

The EU is also looking at mobility and other so-called dual-use technologies. They can be used in military and nonmilitary capacities, such as surveillance satellites that have a role in civil defense.

But EU efforts so far haven’t achieved their potential, say officials. The European Court of Auditors, the EU’s watchdog, recently issued a critical analysis of European military mobility, subtitled: “Full speed not reached due to design weaknesses and obstacles en route."

If spending to close such gaps were included in NATO benchmarks, European military and military-related outlays could get closer to 5% of GDP while enhancing Europe’s readiness to face Russian aggression, officials say.

Yet even with looser rules, many countries will struggle to meet Trump’s goal. Germany, which has Europe’s biggest economy and was long a laggard on arms spending, could offer a test case. Incoming Chancellor Friedrich Merz plans large increases in spending on both defense and infrastructure to start making up for years of underinvestment in both. But even with spending increases that are massive by German standards and looser NATO rules, the country’s new military spending might not reach 4% of GDP, based on current budget discussions.

Write to Daniel Michaels at Dan.Michaels@wsj.com and Laurence Norman at laurence.norman@wsj.com