All references cited in this article are part of the public record. The assertions and conclusions presented, unless otherwise noted, have not been legally contested. Characterizations identified as the author's view are opinion.
For most of NATO's history, the question would have sounded absurd. Then, in January 2026, the President of the United States refused to rule out taking Greenland, a territory of fellow NATO member Denmark, by force. He said the United States would get it "the hard way" if necessary.1 Suddenly the question everyone had avoided was on the front page: What happens when one NATO member attacks another?
The common answer is that no one knows. That is half right. The treaty never imagined an ally attacking an ally, and it has no clause for it. But it is not silent. It says clearly who must be helped, and it leaves each country free to decide how.
What the Treaty Actually Says
The North Atlantic Treaty of 1949 is short. Its heart is Article 5: "The Parties agree that an armed attack against one or more of them in Europe or North America shall be considered an attack against them all." Each ally will then assist "the Party or Parties so attacked by taking forthwith, individually and in concert with the other Parties, such action as it deems necessary, including the use of armed force."2 Article 6 confirms that an attack on the territory of any member in North America counts.2
The treaty does not use the word "aggressor," and it contains no provision for expelling a member or for one member attacking another.2 But notice what Article 5 does define: The party attacked. It does not say "an attack by a non-member." It says an armed attack against a member. As international law scholar Michael Schmitt wrote at the height of the Greenland crisis, "Article 5 imposes no limitations on whom collective defense may be directed against."3 If the attacker happens to be an ally, the obligation to help the victim does not disappear.
The treaty also makes clear who would be in the wrong. Article 1 commits every member to settle disputes "by peaceful means" and to refrain from the threat or use of force inconsistent with the United Nations Charter.2 A member that invades another has broken the treaty before the first shot lands. Nothing in the treaty obliges any ally to help it.
Support Does Not Have to Mean Soldiers
Article 5 is often misunderstood as an automatic promise to go to war. It is not. Each ally takes "such action as it deems necessary," and NATO itself explains that this assistance "may, but does not have to, include the use of armed force."4
That flexibility cuts both ways. It means no ally is forced to send troops. But it also means there are many ways to meet the obligation: Intelligence, weapons, ammunition, logistics, cyber defence, sanctions, diplomatic pressure, and even food, fuel and clothing. Schmitt notes that the help must be offered in good faith and genuinely aimed at restoring security, and that the attacked country must ask for it.3 An ally that simply did nothing would be failing its treaty obligation.
The Catch: Consensus
NATO as an organization acts through the North Atlantic Council, which works by consensus. That means the United States could block any formal NATO decision to defend another ally against it. But the obligation in Article 5 belongs to each member country, not only to the organization. Schmitt concludes that the obligation "could be satisfied through coordinated or unilateral action outside NATO command structures."3 In other words, if NATO's machinery were jammed, the allies could still act as allies.
Has It Ever Happened?
No NATO member has ever invaded another. The closest call before 2026 was the Cod Wars between Britain and Iceland over fishing rights. In the third, in 1975-76, British warships and Icelandic patrol vessels clashed in 55 ramming incidents. Iceland threatened to close the NATO base at Keflavik and hinted it would leave the alliance. The United States helped broker a settlement, and Britain backed down.5
Greenland was the first real test of an ally threatening an ally with force. Denmark's European partners did not wait to find out whether "no one knows" was true. In January 2026, as the threats escalated, Denmark deployed hundreds of Arctic-trained troops, and allies including Britain, France and Germany sent small contingents of their own to Greenland, in what became known as Operation Arctic Endurance.1 The numbers were symbolic. The message was not: An attack on Greenland would mean firing on European allies too. Days later, Trump said he would not use military force, and in September 2026 the United States, Denmark and Greenland signed a security agreement that affirmed Danish sovereignty.16
What It Would Mean for Canada
Canada is in North America, so it is squarely inside Article 5's coverage. If the United States ever attacked Canada, the other members of NATO would be bound to assist Canada, not the United States.
In my view, that assistance would not necessarily mean foreign armies on Canadian soil. It could be military, and France, Britain and Germany are the countries I would personally expect to respond militarily, if any did. They are the same countries that stood beside Denmark over Greenland. But support could just as well come as intelligence, weapons, ammunition, and even food and clothing, keeping Canada supplied and standing.
Australia is not a NATO member, so it would have no treaty obligation. But it is one of Canada's closest partners, a fellow member of the Five Eyes intelligence alliance, and the country now building Canada's Arctic radar. I would expect it to help as a matter of choice, as a friend rather than as a treaty partner.
None of this means Canada could win a war against the United States, and it is fair to ask whether the allies together could. The honest answer is: Not easily, and not in North America. In 2025, the United States spent $954 billion on its military, against $559 billion for all of NATO's European members combined.7 Europe's NATO members have more active troops on paper, roughly 2 million to America's 1.3 million, though that count includes Turkey, whose side in such a crisis is anything but certain.8 The United States holds about 5,177 nuclear warheads, compared with about 225 for Britain and 290 for France.9 But nuclear numbers matter far less than they seem in a war next door. The United States is not about to use nuclear weapons on its own doorstep, where the fallout would drift over American cities as surely as Canadian ones, even if defending forces pushed onto its own soil. Ukraine proved the point. In August 2024, Ukrainian forces crossed into Russia's Kursk region and seized more than 1,000 square kilometres of Russian territory, holding ground for months. Russia, with the largest nuclear arsenal in the world, did not use it.10 A war between neighbours would be decided by conventional forces, endurance and will. And a war in North America would be fought on America's doorstep and an ocean away from Europe. If the United States chose to use its full force against Canada, no realistic allied response could match it on the battlefield.
That is why everything depends on the allies' response, and why deterrence is not about winning. It is about making an attack so costly, so isolating and so damaging to the attacker that it never happens. An America that attacked an ally would face more than soldiers. It would face the loss of trade with its largest partners, the collapse of the alliance that hosts its bases across Europe, and two nuclear-armed allies, Britain and France, whose independent deterrents exist precisely so that no other power can decide their fate. No American government could fight its allies and keep the global position those allies help it hold.
Greenland showed how that works, and its limits. A few dozen European soldiers on the ice could never have stopped an American invasion. They were not there to win. They were there to make sure that any attack on Greenland would also be an attack on Britain, France and Germany. That changed the calculation in Washington. But it only worked because those allies chose to show up. A guarantee is only as strong as the will behind it, which is exactly why the treaty should be modernized to make that response automatic.
Modernizing the Treaty
In my view, the North Atlantic Treaty needs to be modernized for the world Greenland revealed. Two changes would close the gap.
First, say it plainly. The treaty should state outright that allies must immediately protect any member that is attacked, even when the attacker is another member. Legal scholars can argue that Article 5 already means this, but a guarantee that has to be argued is weaker than one that is written down.
Second, cut off the offender's trade, automatically. Any member that attacks another should face an immediate end to trade with every other ally: No imports, no exports, no trade. The reason is as old as war itself. Through the centuries, countries and companies have sold to both sides of a conflict, to hedge their bets or to profit from the misery of both. When the U.S. Senate's Nye Committee investigated the arms industry after the First World War, it found firms that had sold weapons to both sides of international disputes.11 An automatic cutoff would make sure no ally profits from an attack on another, and would give any member tempted to go rogue one more thing to remember.
Greenland showed why it should be automatic. In January 2026, the United States threatened tariffs of 10 per cent, rising to 25 per cent, on eight NATO allies unless Denmark gave up Greenland, using trade as a weapon before any shot was fired. The European Union had a tool built for exactly this, its Anti-Coercion Instrument, and France called for using it. The EU debated it and held back.12 A response that has to be debated in the middle of a crisis is a response the attacker can count on being slow. A response written into the treaty is one the attacker has to price in before deciding to act. And Canada, which is not in the EU, has no such tool at all. A NATO-wide rule would cover it.
Imagine the position of a member that knows, in advance, that attacking an ally means losing trade with all of the others at once. How many non-allies would be willing, or able, to make up the difference? And what backlash would they face from the rest of the alliance if they tried? A cutoff would hurt the allies too, and no one should pretend otherwise. But that shared cost is the point. It makes an attack on one member costly for the attacker in every capital at once. These are clear, concise ways to keep peace and honour between members, should one ever decide to go rogue.
There is a practical obstacle. The treaty has no amendment clause. Article 12 allows members to consult on reviewing it, but changes need everyone's agreement, and a member planning to go rogue would simply refuse.2 The answer is not to wait for unanimity. The other allies can sign a supplementary agreement among themselves, binding each other to automatic defence and trade cutoffs, without asking the permission of the member they are protecting themselves against. The treaty already points in this direction: Article 2 commits members to "eliminate conflict in their international economic policies."2 A member that wages a trade war on its allies, let alone a real one, has already walked away from that promise.
So Who Knows?
The treaty does not spell out what happens when an ally attacks an ally. But it answers the most important question: The attacked member is owed help, and the attacker is owed nothing. What no treaty can answer is whether every ally would find the will to act.
When I say the United States could never win a war against Canada and the rest of the alliance, I do not mean the battle. It might well win the battle. I mean the fallout and the ostracization that would follow in the long term: Allies who would never trust it again, markets closed to it, bases it could no longer use, and a country that had spent eighty years building the most successful alliance in history turning it into a coalition against itself. That is a war no one wins, least of all the side that starts it.
For Canada, the lesson of Greenland is the same as the lesson of every article on this site about the past two years: Build friendships far and wide, so that when it matters, Canada does not stand alone. That is the case this site made in Canada's Military Buildup, and it is why the relationships Canada is building in Europe matter as much as any radar or submarine.