In at least the view of President Trump, his gruff approach to this week’s NATO summit in Brussels was a success.

“Prior to last year where I attended my first meeting,” Trump said of spending commitments by member nations, “it was going down, the amount of money being spent by countries was going down and down very substantially, and now it’s going up very substantially. And commitments were made. Only five of 29 countries were making their commitment and that’s now changed. The commitment was at 2 percent. Ultimately, that’ll be going up quite a bit higher than that.”

NATO Secretary General Jens Stoltenberg would be giving hard numbers for the increase in military spending by member nations, Trump predicted, perhaps at the end of the summit. It was a relief to the president “because the United States has been paying a tremendous amount, probably 90 percent of the cost of NATO.” Now, he said, “countries are going to start upping their commitments.”

As has been noted many times since Trump started making assertions about underpayment on the campaign trail in 2016, his representation of how the system works is not accurate. There are two military spending goals tracked by NATO, as outlined in a 2014 agreement. The first is that member states spend 2 percent of their annual gross domestic product on the military. The second is that spending on equipment make up 20 percent of that military spending. After all, NATO’s ability to engage militarily on behalf of its members necessitates having military assets to deploy.

Most member nations, though, aren’t meeting the 2-percent-of-GDP benchmark. The most recent data from NATO shows that only five of the 29 do, with the recent addition of Latvia. (It’s one of three countries to have passed a law mandating an increase to 2 percent.) Since Trump began advocating for countries to hit that goal, 16 have increased spending relative to GDP by at least 5 percent; six of those countries had already increased by at least 5 percent during the two prior years as well. Eight countries saw less than 5 percent change in the percentage of their spending from 2016 to 2018. Four countries saw the military-spending percentage fall by more than 5 percent since 2016.

Trump floated the idea of spending 4 percent of GDP on military, a mark that no one except the United States is even close to. (In recent years, we’ve fallen below that number.) It’s a bit hard to see how the other 28 member nations might realistically be expected to approach that figure when so few are trending toward 2 percent, but perhaps Trump’s intent is simply to nudge them in that direction.

But this sidesteps an interesting question that probably deserves an answer before evaluating Trump’s insistences: Why is this the metric that’s used anyway? Why is a percentage of GDP the proper way to evaluate the contributions of members nations?

Magnus Nordenman, director for the Transatlantic Security Initiative at the Atlantic Council, was blunt when asked that question in a phone call Thursday.

“Quite frankly, this is a metric that was arrived at by political leadership because it is relatively simple to measure,” he said. “It is a relatively simple metric for the public to understand. So the reason behind it is very much that it’s easy to get, it’s easy to understand and it’s one that is politically acceptable to all of the nations.”

There probably wasn’t a single, unified metric that would make sense for each country, he said. Internally, the alliance “has something they call the three C’s: cash, capabilities and contribution,” Nordenman explained.

That said, he did see value to the GDP metric in two ways.

First, “it sends the signal: Does your nation care about defense? It’s a lot about showing commitment and showing unity and showing shared hardship and shared burden,” he said. “It’s not unimportant to show that I do give a damn, or my nation does give a damn about defense.”

Second, “at some point the only way to do more is have more. To get output, you need input at some point.” In other words, if NATO needs to deploy its resources somewhere, it needs to have the necessary resources to do so.

Polling from Pew Research Center indicates that many citizens of member nations are loath to deploy their own forces on NATO’s behalf while expecting the United States to deploy ours. While it’s not the case that the United States makes up 90 percent of spending — Nordenman indicates we cover about a fifth of the alliance’s annual costs — it is the case that our military is much larger than any other member. (The U.S. military makes up about two-thirds of the total military capabilities of NATO, he said, but the United States is a global force that is dedicated to more than simply the defense of Europe.)

By having more resources to deploy, the sensibility that it’s up to the United States might change — though it’s worth noting that the country whose citizens are most loath to deploy, Greece, is one of the few to meet its NATO commitment.

About that commitment!

“If you read the fine print in 2014,” Nordenman said, “it actually doesn’t even say that allies must reach 2 percent by 2024. It says allies must aim to reach 2 percent by 2024.”

After Trump spoke, Stoltenberg did address the commitment pledges made by NATO members.

“We agreed to make good on the commitments we have made,” he told CNN.

Not exactly what Trump predicted he might say.