Trump left the assembled presidents and prime ministers [at the NATO summit] floundering, unsure whether he was serious about the 4% target, double the existing Nato target of 2%, which many do not meet, or whether it was just a ploy.A percentage of GDP target for defense spending is an entirely arbitrary number. What counts is whether the defense budget is appropriate to a foreign policy that takes into account realistic security risks. And also how well the money is spent. A country with a smaller GDP like Greece - and one that has shrunk incredibly due to the EU's austerity policies - may need to spend more than 2%, but countries like Germany and France maybe less. Greece actually is spending more than 2% now.
After making the announcement, Trump walked out.
The White House press secretary, Sarah Sanders, confirmed the 4% figure. “During the president’s remarks today at the Nato summit he suggested that countries not only meet their commitment of 2% of their GDP on defence spending, but that they increase it to 4%,” she said.
Markus Becker in Gerade noch mal gut gegangen Spiegel Online 12.07.2018 gives the following percentages for the five NATO countries that currently spend more than 2% of GDP on defense:
United States: 3.5%
This is the total percentage for the US, not a percentage spent only on European defense.
Über-Realist Stephen Walt in The EU and NATO and Trump — Oh My! Foreign Policy 07/02/2018 refers to the painfully obvious point that "Trump’s evident distaste for these institutions [NATO AND G-7] mostly reveals his own ignorance and lack of strategic acumen." That's actually putting it generally.
Walt is actually being generous in putting it that way. A desire to join in Vladimir Putin's foreign policy - for whatever reasons - probably also plays a major role. Even though he likely doesn't really understand that either. Not that he would act differently if he did!
Walt gives his realist-theory view of US interests in Europe, which despite myself I have to admit is persuasive as far as it goes:
The U.S. interest in Europe is fairly straightforward. In addition to their mutually beneficial trade and investment relations, the United States has long sought to preserve an overall balance of power in Europe. ...And in line with that view, he thinks that it makes practical sense for the EU nations to maintain cohesion in the Union and take on a bigger role in continental defense. And, "Given that the United States still has an
There is no potential hegemon in Europe today, however — neither Germany nor Russia has the population, economic strength, and military clout to take over the whole place — and thus there is no serious threat to the regional balance of power. Thus, the United States could (and should) reduce its military role and gradually turn European security back to the Europeans. [my emphasis]
interest in a tranquil Europe, a strong EU would be even more valuable if the U.S. security role in Europe were to decline."
But he doesn't think Trump is making any kind of realistic evaluation of US interests in his foreign policy: "The national interest is irrelevant; it’s the Nielsen ratings that count."