Signed in London on 27 May, the UK-Poland Defence and Security Co-operation Treaty is being called ‘NATO within NATO’ by analysts. It is the most significant bilateral defence agreement in Europe since the end of the Cold War and a direct response to Washington’s unreliability as an ally.
By Robert Crawley | 27 May 2026
The ceremony in Downing Street was precise and purposeful. Prime Minister Keir Starmer and Polish Prime Minister Donald Tusk stood before cameras and signed a document that, in the considered language of international treaties, said something quite blunt: Russia is a long-term strategic threat to Poland, to the United Kingdom, and to NATO as a whole, and both countries intend to do something serious about it together. The UK-Poland Defence and Security Co-operation Treaty, signed today, is not a mutual defence guarantee in the Article 5 sense, but it is the most comprehensive bilateral security commitment Britain has made with any European partner since leaving the European Union, and it carries a weight that goes well beyond its legal text.
The treaty covers an expansive range of cooperation. On the military side, it commits both nations to joint development of next-generation weapons systems, including ammunition for air defence platforms and a new medium-range air defence missile being co-developed by British and Polish defence firms. It expands the use of unmanned aerial and ground systems along NATO’s eastern flank, where Poland shares a border with Belarus and the Russian exclave of Kaliningrad. It sets out a programme of joint exercises focusing on counter-drone operations, electronic warfare, and engineering support. And it contains, for the first time in any UK bilateral agreement, an explicit naming of Russia as a strategic threat, not merely a security challenge or an area of concern, but a threat, defined as long-term and explicitly directed at both signatories.
For Donald Tusk, the treaty is the culmination of a strategy he has been building since returning to the Polish premiership in late 2023. Poland has increased its defence spending to four percent of GDP, the highest proportion of any NATO member, and has been systematically constructing bilateral security arrangements with each of its major Western partners. A similar pact with France was signed in November 2025. The agreement with the UK completed what Warsaw sees as a network of committed relationships that can function even if the broader NATO alliance is slow to respond in a crisis, a scenario that Tusk has increasingly named in public as a real possibility given the unpredictability of American policy under the current administration.
“Poland is building a web of security commitments that does not depend on any single partner. That is not distrust of NATO. It is a rational response to the world as it actually is in 2026.”
The treaty is, in part, a product of anxiety about Washington. Since returning to office, President Trump has repeatedly questioned the value of NATO commitments, suggested that European allies should bear more of the defence burden, and on several occasions implied that American military protection is conditional on financial contributions rather than treaty obligations. These signals have accelerated European rearmament and bilateral defence diplomacy at a pace that would have seemed extraordinary even three years ago. Germany has crossed the two percent GDP threshold. France has proposed a European nuclear deterrent framework. And now Britain and Poland have created what the European press is calling a coalition of the willing, formalised in treaty form.
The Kremlin’s initial response was dismissive: a spokesperson described the treaty as ‘an aggressive document that escalates tensions’ and accused Britain of constructing an anti-Russian military infrastructure on Russia’s western border. That framing reflects Moscow’s long-standing position that any NATO-oriented expansion on its periphery constitutes a provocation. What it does not reflect is the reality that this treaty is a response to years of Russian military aggression in Ukraine, a hybrid campaign of sabotage operations against European infrastructure, and the continued occupation of Ukrainian territory. The document names Russia as a strategic threat because Russia has spent several years demonstrating that it is one.
