Around a decade ago.....NATO members sat down and made up this rule. It basically said that whatever your GDP was....you needed to spend 2-percent of that GDP on military requirements.
In recent months, that decision has come up because it's become a problem. Only five nations of the group spend two percent or more on military requirements.
In particular....Germany spends 1.2 percent at present. Naturally, they are viewed upon in a negative position because of this.
The countries making the 2-percent or more? US, Poland, Estonia, UK, and Greece. There are some footnotes to this group....in that Greece is mostly bankrupt and it's hard to explain how they can afford the 2-percent, when Germany can't. In the case of the UK....there are several ways (at least 3) in which you can count GDP, and in one of those methods....the UK fails the 2-percent rule. Naturally, no one in NATO prescribed the correct method.....so it may not matter.
The other reality to this deal? Well....NATO said at the time of this rule (a decade ago) that you'd have until 2024 to reach 2-percent. Will Germany or the others reach the 2-percent? Unknown. Several journalists have written pieces saying absolutely yes. A couple have said that the 2-percent can't be reached in Germany unless taxation occurs or a reshuffling of priorities (cutting infrastructure or people-programs).
Is Trump harping on something to trigger some change? Unknown. I might speculate that he'd like to position US forces from Germany into some other European region and use this lack of spending on Germany's part as a reason. The two countries to get the US forces out of Germany? UK and Poland.
But then I come to this odd problem about this whole discussion. There just isn't much of a threat existing against Europe since the early 1990s. Once the Soviet Union fell apart....the chief reasons for having a large military situation have dissolved year after year. To be honest, no one in Europe really fears much about the Russians except the five or six countries which border Russia and feel occasional intimidation.
So the question ought to be.....is 2-percent a realistic goal or just some fake number? Oddly, no one ever said how you had to spend the 2-percent. So you could go and have an entire military force in your country based on tanks, helicopters, and surface-to-air missiles. You could spend 75-percent of your budget on submarines. You could go and have the bulk of your expenditures on weapons with limited value.
In some ways, I think that NATO has come to the end of it' useful potential. It probably was there at the epic peak of 1991, and the downsizing of the US over the next decade never went to the degree required.
This whole 2-percent thing will be discussed over and over for the remainder of this year and it'll reach a point where the public will ask where the real threat is, and Russia can't be painted up as some massive threat. At that point, I think NATO is finished. It served a great purpose and it's sad that it had to be carved up in some dramatic fashion like this in the end.