Open this photo in gallery U.S. President Donald Trump arrives for a dinner at the Parc du Cinquantenaire during the NATO Summit in Brussels, Belgium, on July 11, 2018. GEERT VANDEN WIJNGAERT/Getty Images

U.S. President Donald Trump has called NATO “obsolete” and scolded other member countries for ignoring guidelines to spend at least 2 per cent of their GDPs on defence.

On Wednesday, he opened a North Atlantic Treaty Organization summit in Brussels by getting into a back-and-forth with Secretary-General Jens Stoltenberg after accusing Germany of being “totally controlled” by Russia because it buys natural gas from Moscow.

“When we stand together, also in dealing with Russia, we are stronger,” Mr. Stoltenberg replied.

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“No, you’re just making Russia richer,” the President interrupted.

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Later in the day, Mr. Trump raised the stakes further in a closed-door meeting, doubling his demanded military spending from other countries to 4 per cent.

Mr. Trump’s rhetorical attacks have left the alliance mulling questions that would have seemed inconceivable when the North Atlantic Treaty was signed in 1949. At a time when the Trump administration has pulled out of international deals, or threatened to, the idea of the United States scaling back its commitment – or even exiting NATO – is no longer far-fetched.

NATO shortfalls in defence budgets Twenty-four of NATO’s 29 members are forecast to fall below thealliance’s long-term target of spending 2% of GDP on defence NATO DEFENCE SPENDING, 2018 est. (% of GDP) U.S. 3.50 Greece 2.27 Estonia 2.14 Britain 2.10 Latvia 2.00 Poland 1.98 Lithuania 1.96 Romania 1.93 France 1.81 Turkey 1.68 Target met Norway 1.61 Montenegro 1.58 Target not met Bulgaria* 1.56 Portugal 1.36 Netherlands 1.35 Croatia 1.30 Germany 1.24 Canada 1.23 Denmark 1.21 NATO spending target of 2% of GDP Slovakia 1.20 Albania 1.19 Italy 1.15 Czech Rep. 1.11 Hungary 1.08 Slovenia 1.01 Spain 0.93 *Does not include pensions Belgium 0.93 Luxem. 0.55 United States Canada $706.1bn $21.6 Britain (second largest after U.S.) NATO total spending $61.5bn US$1,013bn Europe (total) $285.7bn Data based on 2010 prices and exchange rates NATO member Iceland has no standing military force THE GLOBE AND MAIL, SOURCE: graphic news; NATO estimates NATO shortfalls in defence budgets Twenty-four of NATO’s 29 members are forecast to fall below thealliance’s long-term target of spending 2% of GDP on defence NATO DEFENCE SPENDING, 2018 est. (percentage of GDP) U.S. 3.50 Greece 2.27 Estonia 2.14 Britain 2.10 Latvia 2.00 Poland 1.98 Lithuania 1.96 Romania 1.93 France 1.81 Turkey 1.68 Target met Norway 1.61 Montenegro 1.58 Target not met Bulgaria* 1.56 Portugal 1.36 Netherlands 1.35 Croatia 1.30 Germany 1.24 Canada 1.23 Denmark 1.21 NATO spending target of 2% of GDP Slovakia 1.20 Albania 1.19 Italy 1.15 Czech Rep. 1.11 Hungary 1.08 Slovenia 1.01 Spain 0.93 *Does not include pensions Belgium 0.93 Luxembourg 0.55 United States Canada $706.1bn $21.6 Britain (second largest after U.S.) NATO total spending $61.5bn US$1,013bn Europe (total) $285.7bn Data based on 2010 prices and exchange rates NATO member Iceland has no standing military force THE GLOBE AND MAIL, SOURCE: graphic news; NATO estimates NATO shortfalls in defence budgets Twenty-four of NATO’s 29 members are forecast to fall below the alliance’s long-term target of spending 2% of GDP on defence NATO DEFENCE SPENDING, 2018 estimate (percentage of GDP) United States 3.50 Greece 2.27 Estonia 2.14 Target met Britain 2.10 Latvia 2.00 Target not met Poland 1.98 Lithuania 1.96 Romania 1.93 NATO spending target of 2% of GDP France 1.81 Turkey 1.68 United States Canada Norway 1.61 $706.1bn $21.6 Montenegro 1.58 Bulgaria* 1.56 Britain (second largest after U.S.) Portugal 1.36 Netherlands 1.35 NATO total spending Croatia 1.30 Germany 1.24 $61.5bn Canada 1.23 US$1,013bn Denmark 1.21 Slovakia 1.20 Europe (total) Albania 1.19 Italy 1.15 $285.7bn Czech Rep. 1.11 Hungary Data based on 2010 prices and exchange rates 1.08 Slovenia 1.01 Spain 0.93 NATO member Iceland has no standing military force Belgium 0.93 Luxembourg 0.55 *Does not include pensions THE GLOBE AND MAIL, SOURCE: graphic news; NATO estimates

Can Trump pull the U.S. out of NATO? How would that work?

The short answer is yes, but it’s complicated.

Under Article 13 of the North Atlantic Treaty, any country that wants to leave has to send the United States a “notice of denunciation,” which the U.S. would then pass on to the other countries in the alliance. After a one-year waiting period, the country that wants to leave would be out.

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It’s not entirely clear how the United States would notify itself of its intention to withdraw. As the country that started NATO, the treaty’s drafters likely assumed that the United States was one that would never want to leave.

The U.S. Constitution specifies that the president needs two-thirds majority approval of the Senate to enter into a new treaty. But it says nothing about pulling out of one.

In recent decades, two presidents have withdrawn from treaties without the Senate’s backing.

In 1979, Jimmy Carter tore up the U.S.’s mutual defence pact with Taiwan as part of establishing diplomatic relations with the government in Beijing. Members of Congress, led by then-senator Barry Goldwater, mounted a legal challenge. The Supreme Court, however, refused to hear the case, leaving the question unresolved.

In 2002, then-president George W. Bush took similar action, unilaterally pulling out of the Anti-Ballistic Missile Treaty with Russia and three other former Soviet republics.

But even though these actions would give Mr. Trump precedents to point to if he decided to bail on NATO, there would likely still be a political firestorm. NATO has broad bipartisan support in the United States.

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The United States would also face almost complete isolation among its natural allies.

“Even for Trump, raising the prospect of an American withdrawal from NATO is a bridge too far. It would be a geopolitical earthquake,” said Charles Kupchan, a professor of international affairs at Georgetown University in Washington.

Can Trump pull back U.S. involvement in NATO?

As Commander-in-Chief of the U.S.’s armed forces, the President has broad power to decide where and how to use the military.

The United States has more than 60,000 troops stationed in Europe, more than half of them in Germany. Since Russia’s invasion of Crimea in 2014, Washington has poured billions of dollars into the European Reassurance Initiative (ERI), which funds actions meant to protect the continent’s flank from Russia.

If Mr. Trump wanted to send European allies a message, Mr. Kupchan said, he could cut back the ERI or cancel joint military exercises with U.S. allies. “He could do lots of different things,” he said.

Nina Jankowicz, a NATO expert at the Wilson Center think tank in Washington, said any pullback would clash directly with his own actions: Last year, he approved an 18-per-cent increase to U.S. military spending, bringing the total budget to nearly US$700-billion.

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“On one hand, he’s saying, ‘We shouldn’t be paying this much.’ But on the other, he’s saying, ‘Let’s spend more on this than we ever have before,’ ” she said.

What is Trump’s problem with NATO, exactly?

In 2006, NATO members agreed to a guideline that they would spend at least 2 per cent of their GDPs on defence. As of last year, according to World Bank figures, only five of NATO’s 29 members reached that benchmark. Because of the United States’ proportionally larger economy, its defence budget represents 67 per cent of all military spending by NATO countries.

The United States is also the largest contributor to NATO’s $2.4-billion annual direct costs, paying 22 per cent.

Mr. Trump argues this is all unfair, and all NATO members must reach their 2 per cent targets – and even double that.

He says that “many countries owe us a tremendous amount of money” for not increasing their military spending when they said they would.

Sanctions could block Nord Stream 2 pipeline The United States has threatened sanctions against companies in Austria, France, Germany and the Nether-lands that are involved in the Nord Stream 2 natural gas pipeline linking Russia and Germany Nord Stream 2 would deliver extra 55bcm of gas directly to Germany RUSSIA UK GER. POLAND UKRAINE FRANCE SPAIN ITALY TURKEY Main Russian gas export pipelines 500km EU member states Critics fear pipeline would increase Europe’s reliance on Russian gas. Countries such as Ukraine also stand to lose lucrative transit fees EU NATURAL GAS CONSUMPTION BY YEAR Billion cubic metres (bcm) Imported from Russia Other 500 400 300 200 100 0 2005 ‘06 ‘07 ‘08 ‘09 ‘10 ‘11 ‘12 ‘13 ‘14 ‘15 ‘16 ‘17 the globe and mail, Sources: graphic news; Bloomberg, Stratfor Sanctions could block Nord Stream 2 pipeline The United States has threatened sanctions against companies in Austria, France, Germany and the Nether- lands that are involved in the Nord Stream 2 natural gas pipeline linking Russia and Germany Nord Stream 2 would deliver extra 55bcm of gas directly to Germany RUSSIA UK GER. POLAND UKRAINE FRANCE SPAIN ITALY TURKEY Main Russian gas export pipelines 500km EU member states Critics fear pipeline would increase Europe’s reliance on Russian gas. Countries such as Ukraine also stand to lose lucrative transit fees EU NATURAL GAS CONSUMPTION BY YEAR Billion cubic metres (bcm) Imported from Russia Other 500 400 300 200 100 0 2005 ‘06 ‘07 ‘08 ‘09 ‘10 ‘11 ‘12 ‘13 ‘14 ‘15 ‘16 ‘17 the globe and mail, Sources: graphic news; Bloomberg, Stratfor Sanctions could block Nord Stream 2 pipeline The United States has threatened sanctions against companies in Austria, France, Germany and the Netherlands that are involved in the Nord Stream 2 natural gas pipeline linking Russia and Germany Nord Stream 2 would deliver extra 55bcm of gas directly to Germany RUSSIA UK POLAND GERMANY UKRAINE FRANCE SPAIN ITALY Main Russian gas export pipelines TURKEY 500km EU member states Critics fear pipeline would increase Europe’s reliance on Russian gas. Countries such as Ukraine also stand to lose lucrative transit fees EU NATURAL GAS CONSUMPTION BY YEAR Billion cubic metres (bcm) Imported from Russia Other 500 400 300 200 100 0 2005 ‘06 ‘07 ‘08 ‘09 ‘10 ‘11 ‘12 ‘13 ‘14 ‘15 ‘16 ‘17 the globe and mail, Sources: graphic news; Bloomberg, Stratfor

Ms. Jankowicz contends the President is not seeing the bigger picture: The United States gets significant benefits from having troops stationed in Europe, which serves as a launching pad for operations in the Middle East and Asia. What’s more, Hungary, Poland and the Baltic countries all play a role in securing the alliance’s eastern border, creating a buffer with Russia that benefits the West.

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“Quibbling about numbers misses the point,” she said.

