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– President Donald Trump goes to a NATO summit in Brussels on Wednesday with one key talking point: Allies need to pay up on their military spending commitments.

Few policymakers would disagree that Europe needs to spend radically more on its own security, and many European diplomats bashfully admit they have relied too long on U.S. firepower to protect them. At the same time, voices throughout NATO are questioning whether Trump’s singular focus on the bottom line is doing more to hurt the alliance than to help it.

Trump has latched on to the notion that each NATO country should be paying at least 2 percent of its gross domestic product toward its military. Yet diplomats who negotiated that pledge in 2014 say it was never meant to turn into a weapon that could halt U.S. protections for Europe — and they might not have agreed to it if they had envisioned a U.S. president as zero-sum as Trump.

Meanwhile, some military planners emphasize that 2 percent isn’t a magic number that ensures strong defense, and they caution against defining military spending too narrowly. They say in some cases, for instance, it makes more sense to buy high-capacity rail cars than tanks.

“The concern I have is that 2 percent has become a bumper sticker that sort of amplifies the transactional approach of this administration,” said Douglas Lute, a retired U.S. Army three-star general who, as U.S. ambassador to NATO at the time of the spending commitment, was President Barack Obama’s main advocate of the pledge inside the alliance. “It gives them a ready, easy-to-understand measure of transactional pros and cons. And it was never intended that way.”

NATO policymakers have watched with increasing alarm as Trump plays hardball with allies even as he praises such longtime rivals as Russian President Vladimir Putin, whom he plans to meet just days after the NATO summit.

Last month, Trump sent letters to the leaders of many NATO countries warning that continued investment of military resources by the U.S. while allies underspend “is no longer sustainable.” He has directed the Pentagon to review the effect of pulling troops from Germany, a favorite target of his anger.

“Germany, Norway, they’re important allies for us, and yet we’re treating them like deadbeat dads,” said Ben Hodges, who until December was the commander of U.S. Army forces in Europe.

Trump on Monday signaled again that he planned to press allies in Brussels.

“The United States is spending far more on NATO than any other Country. This is not fair, nor is it acceptable … NATO benefits Europe far more than it does the U.S.” Trump tweeted. He went on to overstate the U.S. share of military spending by all NATO members. Trump cited 90 percent; last year, the number was 68.7 percent, reflecting U.S. global ambitions and 3.57 percent of its GDP.

The 2-percent spending figure had long lived as a rough rule of thumb before the Obama administration pushed it as a formal commitment at a 2014 summit in Wales that took place in the shadow of Russia’s annexation of Crimea.

Defense planners, including some of the people who helped develop the 2-percent target, say the figure has less to do with adequately ­protecting NATO than with what might be a realistic goal that increases military spending, even as that spending remains well short of Cold War-era levels.

“It was not a judgment that if everybody spent at 2 percent, then NATO would be fine,” said Adam Thomson, who was the British ambassador to NATO in 2014 and was involved in the push to sign allies on to the pledge. “It was a judgment about what level could be set that was politically at least somewhat ­credible.”

“Nobody could quite have expected the way it has been taken up in such an unsophisticated fashion by Trump, but that, too, has had a real impact,” Thomson said.

Trump officials say the president has needed to use harsh language to jolt NATO out of complacency and scare allies into devoting more money toward defense. They say the U.S. has also dramatically increased funding for European defense, nearly doubling what was spent in Obama’s final year.

“NATO really is making progress, and they are doing it at President Trump’s insistence,” U.S. Ambassador to NATO Kay Bailey Hutchison told Fox News on Sunday.

Opponents of Trump’s single-minded focus point out that the only time NATO’s collective defense provisions have actually been triggered was on behalf of the U.S. ­following the Sept. 11 attacks. And they say that a stable and secure Europe offers a political and economic dividend for Washington.

NATO is expected to increase its overall military spending by 3.8 percent in 2018 — the fourth consecutive year spending has gone up. When Trump took office, only four of NATO’s 29 nations met the 2-percent guideline: the United States, Britain, Estonia and Greece. This year, four more are on track — Poland, Lithuania, Latvia and Romania. About two-thirds plan to commit to get there by 2024.

But those statistics mask wide variation. Greece spent 2.36 percent of its GDP on defense last year, yet much of that goes toward pensions for its retired service members, which serves no defensive purpose. Most of its hardware is devoted toward defense against Turkey, also in NATO.

Ultimately, many diplomats say, Trump’s approach has made it harder to talk about what’s needed for NATO’s security more broadly.

“The whole question of 2 percent has moved out of the realm where you can talk about it in a reasonable way,” said a senior NATO diplomat, speaking on condition of anonymity so that the diplomat’s country would not be next in Trump’s cross hairs. “It’s turned into a dogma. You either believe it or not.”