What is Aukus? It is a new three-way strategic defence alliance between Australia, the UK and US, initially to build a class of nuclear-propelled submarines, but also to work together in the Indo-Pacific region, where the rise of China is seen as an increasing threat, and develop wider technologies. It means Australia will end the contract given to France in 2016 to build 12 diesel electric-powered submarines to replace its existing Collins submarine fleet. The deal marks the first time the US has shared nuclear propulsion technology with an ally apart from the UK. Why did Australia want to change its suppliers? The perceived scale of the Chinese threat in the Indo-Pacific region – a vast zone stretching through some of the world’s most vital seaways east from India to Japan and south to Australia – has grown dramatically in recent years. Nuclear-propelled submarines in this context have longer range, are quicker and are harder to detect. But the UK national security adviser, Sir Stephen Lovegrove, has made it clear Aukus is about more than a class of submarine, describing the pact as “perhaps the most significant capability collaboration in the world anywhere in the past six decades”. He added it was a project “in gestation for some months”. The US president, Joe Biden, spoke of the need to maintain a “free and open Indo-Pacific” and to address the region’s “current strategic environment”. What is China’s response? Relations between the three allies and China were already at a low and the deal, which did not name China but was widely understood to be in response to its expansionism in the South China Sea and aggression towards Taiwan, drew a swift response from Beijing. A Chinese foreign ministry spokesperson said the three countries were in the grip of an “obsolete cold war zero sum mentality and narrow-minded geopolitical concepts” and should “respect regional people’s aspiration […] otherwise they will only end up hurting their own interests”. China also questioned Australia’s commitment to nuclear non-proliferation, while the state-run Global Times, which often takes a harder line than Chinese officials, said: “Australian troops are also most likely to be the first batch of western soldiers to waste their lives in the South China Sea.” When will the submarines be ready and who will build them? No date has been announced, and the scoping phase itself will last 18 months. It is possible America may operate attack submarines out of HMAS Stirling, an Australian naval base in Perth, in the interim. The US will lead the project, and the precise technology it is willing to share is unclear, as is the UK role in the supply of the submarines. How angry is France? Take this tweet from the French ambassador to the US, Philippe Étienne, in the hours after the deal: “Interestingly, exactly 240 years ago the French navy defeated the British navy in Chesapeake Bay, paving the way for the victory at Yorktown and the independence of the United States.” A statement from the French embassy said the decision to “exclude” France “shows a lack of coherence that France can only note and regret” while the French foreign minister, Jean-Yves Le Drian, less diplomatically called the deal “a stab in the back”. Back in 2016, France had described the Australian contract as the deal of the century and the start of a 50-year marriage. It was intended to symbolise a wider Australian-French alliance in the Indo-Pacific that would extend to weapons intelligence and communications. Australian partnership was also central to its 2018 Indo-Pacific strategy. What will be the immediate impact of the deal? Relations between Beijing and Washington look set to continue on their current tricky path, while the western alliance has also been shaken. Emmanuel Macron believed Australian concerns had been assuaged, despite Canberra repeatedly warning France about delays and overruns, but it may also be the US made Australia a deal it could not refuse. However, it also seems apparent the US did not trust Macron on China, since he often said he wanted to steer a middle course between two great powers, speaking of an autonomous Europe operating beside America and China What is the geopolitical significance? It means China faces a powerful new defence alliance in the Indo-Pacific, one that has been welcomed by regional partners such as Japan. It also reaffirms that, after Brexit, the US still wants the UK, and not the EU, engaged as its key military partner. It also gives Biden focus for his post-Afghanistan tilt to Asia. On 24 September, he will host the first in-person summit of the Quad – a bloc involving Japan, the US, Australia and India. Does this breach the nuclear non-proliferation treaty? The nuclear watchdog the IAEA says it will investigate, but six countries already use the technology to power their submarines.