NATO is a 28-nation alliance built on the premise that an attack on one member is an attack on every member. That’s a healthy dose of reassurance in a world where terrorism and Russia pose menacing threats. Yet 23 of those countries don’t pay their fair share for that NATO protection.

The U.S. bears most of the burden for propping up NATO, a longstanding disparity previous administrations, including Barack Obama’s, have groused about but never tackled. Until last week, when the Trump team gave its allies a sobering ultimatum. U.S. Defense Secretary Jim Mattis showed up in Brussels to give notice to the laggard nations: Pull your weight or risk a White House decision to scale back America’s commitment to the alliance. “No longer can the American taxpayer carry a disproportionate share of the defense of Western values,” Mattis told his counterparts at a NATO defense ministers meeting. “Americans cannot care more for your children’s future security than you do.”

Well said, Secretary Mattis. It’s a line of thinking that echoes the remarks that President Donald Trump has made about NATO that make sense, and ignores the ones that don’t. Trump is dead wrong in his repeated labeling of NATO as obsolete. “It’s obsolete because it wasn’t taking care of terror,” Trump said in a joint interview with the Times of London and Bild, a German newspaper, just days before taking office.

Has Trump forgotten — or is he unaware of — the only time Article 5 of the North Atlantic Treaty has been invoked? Article 5 says an attack against one ally is regarded as an attack against all. It was used for the benefit of the U.S., after the 9/11 attacks. NATO nations fought alongside the U.S. in the battle against al-Qaida and the Taliban in Afghanistan. And they’re training fighters in the Middle East battling the Islamic State. That’s called “taking care of terror.”

But Trump is right to criticize the lopsided burden shouldered by the U.S. NATO asks member nations to spend at least 2 percent of their annual gross domestic product on defense. The aim is to ensure that the cost of providing military capability is spread evenly among members. Right now, no country comes close to America’s contribution. In Afghanistan and every other NATO endeavor, the U.S. role always dwarfs the troop and resource commitment made by its allies. In 2016, the U.S. spent about $664 billion on defense spending, more than double what the other NATO nations spent on defense combined.

In fact only five countries — the U.S., Britain, Poland, Estonia and, yes, debt-ridden Greece — meet the 2 percent mark. The spongers include countries with robust economies, including France (1.78 percent), Germany (1.19 percent) and Canada (.99 percent). The U.S. commitment: 3.61 percent.

Getting every country to the 2 percent mark won’t be easy, especially at a time when European Union mandates compel member nations to trim their budget deficits. But any country seeking incentive only needs to look east — toward the Kremlin and its desire to put Eastern Europe back under its wing.

Russian President Vladimir Putin has pilfered parts of Ukraine and has heavily fortified the Russian exclave of Kaliningrad, sandwiched between NATO nations Poland and Lithuania. Russia has also reportedly deployed a ground-launched cruise missile system in violation of a 1987 treaty that bans U.S. and Russian land-based intermediate range missiles.

Mattis alluded to facts on the ground: “Events of 2014 were sobering,” he said, referring to Russia’s 2014 annexation of Crimea. Non-compliant Germany seemed to get the message. In an op-ed in a German newspaper, the country’s defense minister, Ursula von der Leyen, wrote, “Germans and most Europeans have for too long relied on the broad shoulder of our American allies.”

Recognition is always the first step. Now for Step Two: Ante up.