Donald Trump, who has long criticised the international alliance, has never shied away from voicing his concerns — namely that many of them aren't pulling their weight financially.
But it's Germany that's really got him riled up at the moment, as you can see in this rather uncomfortable snap of him with Chancellor Angela Merkel.
There's two key reasons for that disgruntled pout. One is Trump's belief that the United States spends too much to uphold NATO compared to the other members.
The other is over a giant pipeline construction plan between Germany and Russia. Here's the lowdown.
The North Atlantic Treaty Organisation, or NATO, is a group of 29 European and North American countries that vows to protect its members.
German Chancellor Angela Merkel and US President Donald Trump haven't seen eye to eye. Photo/Getty Images.
The organisation was founded in 1949 to prevent conflict and combat the Soviet Union, under the idea that a rival power would face a defensive network if it attempted to attack any one of the 29 countries.
But Donald Trump takes issue with the organisation. He believes America has invested too much of its money into the group, while other countries aren't "pulling their weight".
Billions of additional dollars are being spent by NATO countries since my visit last year, at my request, but it isn’t nearly enough. U.S. spends too much. Europe’s borders are BAD! Pipeline dollars to Russia are not acceptable!
During a two-day NATO summit that began on Tuesday, Trump met NATO Secretary General Jens Stoltenberg, and he was quick to lash out.
"Many countries are not paying what they should," the President said publicly. "And, frankly, many countries owe us a tremendous amount of money for many years back, where they're delinquent, as far as I'm concerned, because the United States has had to pay for them."
He said this problem has "gone on for decades", but credited himself as the first president to bring it up.
Is Trump right?
Trump has made a number of arguments on America's role in NATO. Here's a breakdown:
Argument 1: 'Many countries are not paying what they should.'
This is true. What "should" they be paying? We can't measure this with a raw number — there's too great a difference between each country's gross domestic product (GDP).
Rather, let's think of this in percentage terms. NATO's current guideline for every country is to spend at least two per cent of their total GDP expenditure on defence.
Only four countries — the US, Britain, Estonia and Greece — are exceeding this recommended target, with the US spending the most at over 3.5 per cent, according to last year's figures.
A few countries, including Poland, Latvia and France, are close to the target, while others — particularly Luxembourg, Belgium, Spain and Slovenia — are falling well short of it at less than one per cent.
Here's a nifty little graph that sums last year's spending:
Argument 2: 'NATO's allies owe us a tremendous amount of money.'
This is false. Despite the shortcomings of the countries listed above, there is no such thing as debt to the US or NATO, or reimbursing allies.
Trump's argument here is that the US military has always had to spend more to compensate for the NATO countries that spend less. But to suggest the US can "bill" other countries for filling the gap in previous years is ludicrous. That's just not how an alliance works.
"We are asking NATO allies to invest more in their OWN national defence budgets. There are no past dues. No one is delinquent. It's not a country club," said Julie Smith, a European security expert who worked under the former Obama administration.
Argument 3: 'You know, we're protecting Germany, we're protecting France. We're protecting everybody. And yet we're paying a lot of money to protect.'
If we strip this right back, it's kinda true. The US does spend a lot more than its allies on defence. NATO itself acknowledges an "over-reliance" on the US.
But to suggest the US is giving away all this free money and getting nothing in return is false. NATO partners argue that the US gets increased global security out of its spending. Trump does not acknowledge that NATO partners are expected to defend the US if it's threatened.
Article 5, the collective defence portion of the North Atlantic Treaty, has been invoked just once — following the 9/11 terror attacks.
Mr Stoltenberg also argued that the US military presence in Europe, based on its NATO membership, gives it an advantage in dealing with Russia and projecting power in the Middle East and Africa.
What about Germany and that map?
Remember how Trump said "many countries are not paying what they should"?
Based on NATO's statistics, Germany falls squarely into this category. The 29 countries' median defence expenditure is 1.3 per cent. Germany sits at around 1.2 per cent.
Germany is Europe's most prosperous country, and the second-largest contributor of troops to NATO missions behind the US. For what it's worth, Ms Merkel has committed to reaching the 2 per cent threshold by 2024.
But now in comes the Nord Stream 2 pipeline — ie that eye-catching map which made you click on this story in the first place.
Trump hasn't held back in his criticisms of Germany at recent meetings. Photo/News.com.au.
Berlin has approved and started building a 1300km pipeline beneath the Baltic Sea, which will extend between Germany and Russia.
Germany is dependent on Russia for gas, serving as its biggest export market, and this new project — which is expected to be completed by 2020 — will double this risk.
The pipeline is being built by Gazprom, an energy giant owned by the Russian Government. Trump fears this will give Moscow increased influence over Berlin.
Can Western Europe trust Russia? Even if it doesn't, is engaging with Moscow a better or riskier strategic move? Does working with the Russian Government on such a big project pose too great a security risk? Will this project drive a wedge between Germany and other Western nations?
These are some of the big questions being asked, and all in all, Trump is not happy that Chancellor Angela Merkel is proceeding with the construction.
This leads us back to Germany's NATO spending. "We're supposed to be guarding against Russia, and Germany goes out and pays billions and billions of dollars a year to Russia," Trump said.
Earlier today he tweeted:
What good is NATO if Germany is paying Russia billions of dollars for gas and energy? Why are there only 5 out of 29 countries that have met their commitment? The U.S. is paying for Europe’s protection, then loses billions on Trade. Must pay 2% of GDP IMMEDIATELY, not by 2025.
He accused Germany of being a "captive" of Russia, suggesting its actions were hypocritical.
Why should we care about this?
The validity of Donald Trump's concerns depends on who you ask.
"America First" is a staple of his presidency, and his supporters tend to share his belief that other NATO members need to up their game if the US is to remain a member.
Some analysts argue Trump has long demonstrated a problem with Germany, from openly criticising Ms Merkel's immigration stance to complaining about too many German cars on American roads.
According to Politico, Trump may believe that Germany has taken advantage of the US and its key role in Europe's economic union.
Germany also argues Trump's motivation in countering the pipeline is getting the European country to rely more on US energy exports.
Meanwhile analysts argue it's more personal than that; that Trump takes issue with Ms Merkel specifically because she was one of former president Barack Obama's favourite foreign leaders.
What do these bubbling hostilities mean for the future of NATO?
While it's too soon to say, there's a fair chance the organisation would collapse if the US was to leave.
This may compromise the security of Western nations — not to mention create a radical shift in the world's global set-up.
There's plenty of evidence to suggest that authoritarian nations like Russia continue seeking to undermine the unity of Western nations, and Moscow would love nothing more than the separation of the US and Western Europe.
Even the US navy hinted its opposition to Trump's stance, tweeting in support of America's role in the organisation just days ago:
At the same time, experts argue the organisation allows the West to maintain a unified global presence and preserve the freedom of the seas, which is good for global security.
