ONLY SIX countries in the world—America, Britain, China, France, India and Russia—currently operate nuclear-powered submarines. Australia may become the unlikely seventh. In a statement and joint televised appearance on September 15th, Joe Biden, Boris Johnson and Scott Morrison, America, Britain and Australia’s leaders, announced what they described as an “enhanced trilateral security partnership”, awkwardly named AUKUS. Its first initiative, and the jewel in its crown, will be collaboration on future nuclear-powered submarines for the Royal Australian Navy. The pact, which will be signed formally in Washington next week, reflects their shared concern over China’s growing power, and America’s eagerness to beef up the military capabilities of its Asian partners.

AUKUS is based on an Australian idea. It will cover diplomatic, security and defence co-operation in the Indo-Pacific. It includes joint work on cyber capabilities, artificial intelligence, quantum technologies and “additional undersea capabilities”, such as underwater sensors and drones.

Whatever the practical implications of the pact, its symbolic importance was sufficient to provoke China into immediate denunciation. A Chinese foreign-ministry spokesman, Zhao Lijian, said that it seriously damages regional peace and stability, intensifies the arms race and, for good measure, undermines the non-proliferation treaty. Allies of the three parties to the pact were correspondingly enthusiastic about AUKUS. In Japan, Kato Katsunobu, the chief secretary to the cabinet, hailed its significance for peace and security in the region. In Taiwan, the country with most immediate reason to fear China’s intentions, a presidential spokesman welcomed it as part of a “positive and necessary trend for peace and stability in the region”.

Its most immediately eye-catching element is the sub deal, which is thought to be the most significant international collaboration on defence capability anywhere, for decades. Australia had previously signed a $90bn contract with Naval Group, a French company, to build a dozen advanced diesel-electric submarines, but had grown frustrated at the firm’s failure to invest enough in local suppliers. It is now ripping up that deal, to French fury. Jean-Yves Le Drian, the foreign minister, accused Australia of a “stab in the back”, and the type of “brutal, unilateral and unpredictable decision” associated with Donald Trump.

Instead of honouring the French contract, Australia will acquire nuclear subs and its partners will be America and Britain, both of which have operated such vessels for decades. “We will leverage expertise from the United States and the United Kingdom, building on the two countries’ submarine programs to bring an Australian capability into service at the earliest achievable date,” promised the joint statement. Some Australian newspapers have reported that America may operate attack submarines out of HMAS Stirling, an Australian naval base in Perth, in the interim.

The acquisition of nuclear-powered submarines would significantly bolster Australia’s navy. They are larger and pricier, but they are also faster and can stay under the water for far longer than diesel-electric ones, like Australia’s current Collins-class submarines (pictured), which need to surface periodically. They can also go longer without being resupplied, an important factor in the sprawling Pacific. The Centre for Strategic and Budgetary Assessments (CSBA), an American think-tank, calculates that whereas a diesel-electric submarine sailing from Perth could remain “on station” for 11 days in the South China Sea, a nuclear sub could do so for more than two months.

The proposed new vessels would thus provide “real… striking power”, says Malcolm Davis of the Australian Strategic Policy Institute (ASPI), “which is what we need in deterring and responding to a growing challenge from China’s PLA [People’s Liberation Army]”. Australia’s relationship with China has grown increasingly frosty. Last year China imposed bans on various Australian goods in response to its calls for an inquiry into the origins of the coronavirus pandemic.

The new partnership also comes at an opportune time for Mr Biden. His withdrawal from Afghanistan, and the ensuing collapse of the Afghan government, caused concern among many allies about America’s reliability. In theory, that withdrawal was part of a broader reorientation of American diplomatic and military resources to Asia. In practice, many allies there have been sceptical. “The Biden administration’s approach to the Indo-Pacific has so far lacked focus and urgency,” complained a report by the United States Studies Centre (USSC) at the University of Sydney last month.

Ashley Townshend, the co-author of that report, says that Mr Biden’s willingness to share highly advanced defence technology—“something the US has rarely been willing to do”, he notes—is a welcome surprise. “It suggests a more strategic approach to collective defence.” On September 24th Mr Biden is also due to host the first-ever in-person summit of the leaders of the Quad, a burgeoning diplomatic bloc that includes America, Australia, Japan and India.

Yet nuclear co-operation between America, Australia and Britain is not without its problems. America’s navy is “perennially short of submarines at the moment, with the situation likely to deteriorate before it gets better”, says Phil Weir, a naval expert. American and British capacity to build nuclear reactors is also stretched thin, he says. Building up additional capacity to support an Australian programme will take years. The leaders’ statement on September 15th said that an “initial scoping phase” would last 18 months. In 2017 Marise Payne, then Australia’s defence minister and now its foreign minister, acknowledged that a “sovereign” nuclear fleet would take “far longer than a decade”, and would come “at a very substantial cost premium to our conventional fleet”.

Nuclear power also has wider strategic implications. Although the nuclear non-proliferation treaty bars non-nuclear-armed signatories from making bombs, it does—in what amounts to a loophole—allow them to remove nuclear material from formal international oversight if it is for a submarine. The enriched uranium in submarines, however, is the same as that used in a bomb. Worse still, the fuel used in both British and American submarines is enriched to especially high levels.

Though Australia is unlikely to want a nuclear bomb itself—it gave up its pursuit of nuclear weapons in 1973—other nuclear-curious countries may see a submarine as a convenient route to bomb fuel. Brazil is working on its own nuclear sub, which it hopes to commission in the 2030s, and Iran has toyed with the idea in the past. South Korea, which this week tested a submarine-launched ballistic missile from a conventionally powered submarine, will also be watching closely. Australian submariners, meanwhile, will be popping corks and pulling out their physics textbooks.

Editor’s update (September 16th 2021): This story was updated to include details of other governments responses to the deal