An attack on Britain would not test whether Nato exists on paper, but whether it still exists in practice
For most of the post-war period, the question of who would come to Britain’s aid in a crisis barely needed asking. Nato’s Article 5, backed by American power, was assumed to be both automatic and overwhelming.
That assumption is quietly eroding. Recent US actions – from the overnight capture of Venezuela’s president to threats against Greenland, a Nato ally (“if we don’t do it the easy way, we’re going to do it the hard way,” as President Donald Trump put it last week) – expose a deeper uncertainty: American support for international law and treaty obligations increasingly appears optional. Certainty has been replaced by ambiguity.
Meanwhile, Russia has learned to operate below the threshold of declared war, testing alliances through “grey-zone” actions like cyber attacks, sabotage, and calibrated strikes – actions designed to blur legal and moral responsibility and delay any response. It is waging what some analysts call a “shadow war”. Nato works best when aggression is clear, attributable and undeniable. Russia’s incentive is therefore to stay just below that line.
In this world, the most important question for the UK is no longer what its allies are legally obliged to do should Russia strike against us, but who would actually act – and how far they would be willing to go.
Grey-zone actions are not simply minor cyber nuisance or routine sabotage. They can be violent, visible and highly disruptive – but deliberately designed to stay below the threshold of full-scale war. One recent war-game run by Sky News imagined Russia falsely blaming the UK for an attack on a Russian military base, then launching limited missile strikes on targets such as RAF Fylingdales in North Yorkshire (our ballistic missile early-warning system), GCHQ and a civilian site in London.
The objective was not conquest but coercion: to expose UK vulnerabilities, punish us symbolically and force a decision by the US and European allies on their support for Article 5 in practice. Every provocation tests the viability of Nato’s response, and Russia learns more each time.
Even without US ambiguity, Nato’s protections are least useful precisely where modern conflict is most likely. Article 5 remains the alliance’s cornerstone, but it is not automatic. In a grey-zone scenario, attribution is contested, proportionality debated and escalation feared. Article 5 guarantees consultation, not the cavalry. In a world where the US treats treaty obligations as optional, certainty gives way to hesitation – and hesitation invites further probing.
In this situation, support would depend less on treaty language and more on who has most to lose – not just in that specific conflict, but also if Nato’s guarantee was weakened. Eastern Europe, under direct pressure from Russia on Nato’s eastern flank, sees the alliance as existential. With strong political will to resist Russia, and credit earned from the UK’s support for Ukraine, they are likely to help.
Western and Southern Europe (plus “Five Eyes” intelligence partners like Australia and Canada) are at less immediate threat and would see stronger domestic constraints against joining a war – with the chilling effect particularly strong if the US does not intervene. Their support may be partial (e.g. intelligence sharing or equipment rather than deploying troops), though a baseline of ongoing conventional cooperation should keep them responsive – such as combined exercises, expeditionary forces and shared operational planning with France. For them, Nato credibility matters, even if its collapse would not be immediately fatal.
Then there is the ultimate guarantor: the United States. Defence and intelligence structures provide deep institutional support embedded over decades of cooperation. But political leadership – the level that matters most – is far less predictable, with Trump and his advisors increasingly treating European security transactionally.
The paradox is stark. Those most willing to help are least able, and those most able – above all the US, whose intelligence, satellites and communications are critical – are the most uncertain to act. Britain’s risk is not that it will be abandoned, but that it will receive fragmented, deniable and insufficient support that fails to achieve a clear victory and restore deterrence.
Russia would likely target Britain’s most exploitable “bruises”: gaps left by decades of post-Cold War defence cuts, including relatively thin air and missile defences, concentrated national infrastructure and politically sensitive civilian disruption. Grey-zone strikes aim to generate pressure for restraint before alliance unity hardens, to stop the momentum for retaliation before it starts.
As Russia knows, grey-zone responses are cumulative. Early hesitation encourages further probing. If Nato fails decisively once, it becomes less likely to respond in the future. Deterrence erodes over time, not in a single moment. The Russian playbook in Ukraine demonstrates this: disinformation, deniable troops, limited strikes, then invasion.
Britain would not be tested for military mass alone. Its political leadership matters: it is widely seen, especially in the Baltics and Poland, as the backbone of European resistance. Our clarity, willingness and consistent support for allies – rather than sheer force – make us a rational test case. Undermining the UK would disproportionately weaken both European resolve and the Nato guarantee.
The consequences extend beyond Europe. A transactional, rules-optional approach may deliver short-term leverage for the US, but it also empowers authoritarian states comfortable breaking norms – Russia in Ukraine, China over Taiwan – and raises the risk of miscalculation.
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The strongest power may gain most early, but also lose most over time as stability decays; once rules no longer bind the strongest power, they bind no one. Over time, the return to the “spheres of influence” politics destabilise the global order, a lesson painfully reinforced by the two world wars of the 20th century.
A grey-zone attack on Britain would not test whether Nato exists on paper but whether it still exists in practice. In an era where ambiguity replaces certainty, deterrence rests less on treaty text than on political will, speed of response and the confidence that allies will act before hesitation turns into precedent.
That is the test Russia would be setting, and the one Britain could well face first. That this debate now feels necessary shows just how much the Trump administration has changed the strategic calculus.