President Donald Trump will attend the first NATO summit of his second term Tuesday, at a time when the alliance’s 21st-century relevance has never been clearer — but when its sustainability is also in question, with a NATO-skeptical American president back in the Oval Office. Given these realities, Europe needs to show it will contribute its share to ensure the alliance is stable and strong.

But, days before the summit, Spain rejected a proposal from NATO Secretary General Mark Rutte for alliance members to commit to eventually investing 5 percent of their gross domestic product in defense-related spending. To avoid derailing the summit, Rutte agreed on Sunday to exempt Spain.

Spanish Prime Minister Pedro Sánchez, a socialist, said that 5 percent “would be incompatible with our welfare state and our world vision.” Then he added that his country remains “fully committed to NATO.”

No doubt, Sánchez is fully committed to the benefits of NATO membership. It’s the most effective, powerful and vital military alliance in the history of the world, providing the protection of the American nuclear umbrella and much more. But that comes at a cost, and the United States cannot bear a disproportionate share. (U.S. defense spending accounts for about half of all NATO spending.) The Ukraine war demands that alliance members deter a revanchist Russia, which will require broad investment in European self-defense.

Sánchez complains that increasing defense spending might require trimming some public benefits, scaling back green energy projects or taking on debt. His reluctance reflects the attitude, explicit or implied, among many European NATO members for many years.

Trump helped force European allies to take their own defense more seriously — especially Germany. But Spain’s position threatened the Pan-European rearmament effort, because NATO bylaws require consensus among all 32 member states. A joint statement, to be signed at the end of the abbreviated summit in the Netherlands on Wednesday, will now call on “allies” — instead of “we” — to reach 5 percent.

Rutte’s plan already offered NATO members lots of wiggle room. The 5 percent topline includes 3.5 percent for core defense spending — tanks, planes, missiles, troops — and allows for 1.5 percent to go toward “military-adjacent” initiatives. This could mean investing in infrastructure, such as roads and bridges — or even cybersecurity resilience.

The NATO proposal would allow a country’s aid to Ukraine to count toward its core defense spending target.

Last year, Spain spent less on defense than any other NATO member despite being the eurozone’s fourth largest economy. This spring, Sánchez announced that his government would raise defense spending by $12 billion in 2025 to reach a preexisting target of 2 percent of GDP. Alas, even that modest improvement prompted blowback from the left flank of his coalition. With his inner circle and even members of his family ensnared by corruption scandals, some of Sánchez’s allies have publicly pressed him to call early elections. Last week’s announcement was aimed at placating these people.

But politics does not end at the Pyrenees. Spain’s obstinance gives Trump fodder to call into question the U.S. commitment to the broader alliance. And it gives other European countries an excuse to drag their feet down the road.

Also in question is the time by which countries need to reach any new spending target. Rutte initially floated 2032, but the final document might now wind up saying 2035 — or not setting a date at all. The former Dutch prime minister has said that Russia could be ready to launch an attack on NATO territory by 2030 — a reason to be more aggressive.

Spain isn’t the only country that needs to boost security spending. Canada spent 1.3 percent of GDP on defense last year, though Prime Minister Mark Carney has pledged to hit 2 percent this year.

Italy and Belgium are still not on track to meet the 2 percent target that NATO agreed to in 2014. Britain has, so far, committed to spending just 3 percent on defense by 2034.

Trump is right that Europe needs to become more self-sufficient. While the U.S. commitment to NATO should remain ironclad, Washington must also make major investments in other theaters. The strike on Iranian nuclear facilities over the weekend shows the U.S. will be unable to avoid the Middle East. And, most urgently, America needs to counter a rising China.

From London and Brussels to Ottawa, NATO is worth fighting, and spending, to preserve. That truth should be visible in Madrid, too.