The Chinese defence ministry later said the two nations had already been in contact about the drone, and that Mr Trump's "inappropriate and unhelpful" intervention had "unilaterally hyped up the issue". The Chinese navy had merely "adopted a professional and responsible attitude in investigating and verifying the device".

Putting aside the diplomatic ruffles, the more interesting aspect of this incident is the drone itself.

Air power played a critical role in 20th-century warfare, and sophisticated aerial drones have become a prominent military tool of the 21st century, used both to gather intelligence and to attack targets.

However, little public attention has been given to the role marine drones (or unmanned underwater vehicles) are likely to play in future naval warfare. The Pentagon says the small glider captured by China's navy was collecting unclassified data "such as salinity, water temperature and sound speed". Regardless of whether this is true, the US Navy is also developing marine drones for military purposes, including surveillance, hunting other ships and firing weapons. The advantages are clear: these vehicles won't require crew and life-support systems, will be quieter and will have a longer range.

This raises questions about what Prime Minister Malcolm Turnbull called "the largest and most complex defence acquisition Australia has ever undertaken": the government's decision to build 12 submarines, at a cost of about $50 billion. These submarines may begin to enter service in two decades' time, though their construction will not be completed until the 2040s. And that's assuming the project runs to schedule, which, given the Australian Defence Force's record, is a bold assumption.

The ADF is, of course, aware of the looming changes in marine warfare. Its latest white paper notes that some Asian nations will, even before Australia's new submarines are built, acquire "more autonomous systems, such as unmanned combat vehicles", including for underwater operations.

Many critics says the submarine program is little more than an extraordinary costly scheme to subsidise local industry and jobs. Given the military's perpetual problems with wasting money, it's hard to dismiss such concerns. Last week's revelation, via the Auditor-General, that the Defence Department couldn't explain why it spent $200 million more than it had budgeted on services for ADF bases only furthers the impression that the top brass in the Russell Offices work to their own rules, regardless of the efficiencies demanded from the rest of government.