Late last week there was an interesting and potentially very dangerous development in China’s ongoing bullying behaviour at sea in the Far East. An Anzac-class frigate of the Royal Australian Navy stopped to conduct diving operations (to clear fishing nets that had fouled its propellors) in international waters off Japan when it was approached by a destroyer of the People’s Liberation Army Navy (PLAN).

The Australian ship, HMAS Toowoomba, then called PLAN ship Ningbo to inform them that they had divers down and asked them to keep their distance. So far so normal.

In response to this, it appears the PLAN destroyer closed the Toowoomba and turned on her bow-mounted sonar, putting enough sound energy into the water to injure the Aussie divers. A spokesperson for the Australian Defence Minister said, “medical assessments conducted after the divers exited the water identified they had sustained minor injuries likely due to being subjected to the sonar pulses from the Chinese destroyer.”

The reason why this is so dangerous is the nature of active sonar (as opposed to passive sonar, which is for listening only). The sonar, in this case mounted under the bow of the warship, ‘pings’ a large amount of sound into the water to locate submarines or other objects of interest. The sound energy then bounces back off whatever it encounters, be that the sea bed, sea life – RN sonars are immediately turned off if marine mammals are spotted – layers or other phenomena in the water itself or, ideally, the submarine you are looking for.

Imagine the fraction of the Sun’s energy that hits the earth and then how much of that fraction makes it way back to the Sun and you get an idea of why so much power is needed. Anti-submarine warfare sonars are easily powerful enough to kill an unprotected human who gets too close.

In this case, either the PLAN have done the sums on how much power to put into the water at their chosen range to ‘injure not kill’, they were guessing or they had no idea what effect it could have. I’ll let you decide which is more worrying.

This is a long way from the first example of PRC ships behaving in a belligerent manner. Firing water cannons, ‘riding off’ – that is, shoving other vessels off course – and more recently aiming blinding lasers at ships bridges and aircraft have become commonplace in the last few years. But most of the time their maritime thuggery has been enacted by their coastguard or some of their special non-fishing fishing vessels. The bullying has also mainly been directed at patrol and resupply boats that often lack the ability to shoot back. It’s obvious how a warship-on-warship interaction of this sort increases the likelihood of escalation.

Command decision making in the Toowoomba would have been interesting during this encounter. You’ve been told throughout your deployment to make sure you de-escalate, and you have a firm grasp of the strategic situation in which you are operating. On the other hand, they’re deliberately pinging, risking the lives of the divers you have down. It’s classic grey zone stuff, specifically designed to make a conventional response difficult. Clearly you’re going to get back on the radio telling them in much stronger terms to stop using their sonar. But what else? Searchlights, sounding sirens, uncovering weapons, crewing and then training them – all standard escalatory measures. If they had fired a burst of small calibre ammunition in the general direction of your diving boat – a reasonable comparison – you would almost certainly fire back invoking your inherent right to self-defence. Or would you? Your shafts are locked so that they can be dived upon and so you can’t move, much less manoeuvre. You’d be a sitting duck if it escalated further. These are the sort of command decisions that you get paid for in that instant that often get lost in the subsequent strategic discussions over who is condemning who.

It has been clear for some time now that China is attempting to turn the South China Sea into a militarized backyard in which the price of entry gradually increases until it becomes unacceptable. Less clear is why Beijing is accelerating this process at such a rate when Putin’s latest invasion of Ukraine provides a live case study into the unifying effect moving too fast can have. It can only be because they recognise that US resources are already creaking under the weight of dealing with so many global situations.

So what next? Interestingly the Australian Prime Minister and President Xi Jinping were both at the Asia-Pacific Economic Cooperation (APEC) forum just after the incident. It isn’t clear if PM Albanese took the opportunity to deliver his government’s line that it was “dangerous, unsafe and unprofessional” and that “communications guardrails between militaries are now needed” in person. It’s possible that Xi’s line “to urge the Australian side to respect the facts and stop making reckless and irresponsible accusations against China” was delivered in riposte at the same time but this seems equally unlikely.

Meanwhile, out at sea there needs to be a clear and demonstrable shift in the Rules of Engagement (ROE) for all navies operating near China that the use of active sonar in this manner constitutes a hostile act and will be met with proportional force. No commanding officer wants to be the one that starts ‘the war’ so having political and legal backing from home command helps a great deal if you end up being the one who has to take action because you believe the lives of your ship’s company are suddenly at risk.

The other two ‘solutions’ – alliance building and Freedom of Navigation Operations (FONOPS) – have been in train now for some time under US overwatch. FONOPS in particular remains the best lever to demonstrate that increasing attempts to breach internationally recognised operating norms at sea are unacceptable. The more allies that can do this the better as it demonstrates unity but also eases the resource stretch that the US Navy is currently suffering – arguably one of the reasons the People’s Republic of China (PRC) under Xi’s direction are accelerating their grey zone activities at such a rate just now.

‘Grey zones’, especially ones with elements in them as dangerous as this, can become ‘military zones’ at the turn of a missile key. The PRC leadership needs to be reminded of this, backed by continuous demonstrations of allied determination and proportional strength.

If this can’t be done, sooner or later the shooting will start.