Australian strategic thinking has long struggled to move beyond a narrow view of defence that focuses solely on protecting our shores. However, in today's world, our economy could be crippled without an enemy boot stepping foot on Australian soil.

Australia's acquisition of nuclear-powered submarines through AUKUS marks a shift in this mindset.

It is not a strategy in itself, but a structural pivot: a recognition that our vital interests lie far beyond the coastline, and that defending them requires Australia to project its maritime power.

Over a century ago, US naval officer Alfred Thayer Mahan observed that "wars are won by the economic strangulation of the enemy from the sea".

While not universally true, this maxim is directly relevant to an island nation like Australia - 99% of our international trade moves by sea.

But not just any trade - our critical supplies of fuel, fertiliser and ammunition all come by sea. Australia's economy and defences would be crippled if these things were stopped at sea.

These vulnerabilities are compounded by our growing dependence on undersea cables for communications.

Strategic concepts that rely on making Australia's territory a hard target, such as the "strategic defensive", fail to grapple with this reality, perpetuating a flawed understanding of how to defend Australia.

Viewing Australia's interests solely through the lens of avoiding or defeating a territorial attack overlooks the reality that an adversary could cripple the nation far more easily through the maritime, space or cyber domains.

The ability to project power in the seas and oceans far from Australia's shores is critical to protecting these seaborne supply lines and sustaining the national economy. This is where AUKUS comes in - the endurance and range of nuclear-powered submarines are a key element.

Australia's future nuclear-powered submarines would make adversary naval task groups vulnerable if they threatened our maritime trade routes.

Much more is needed, however, to deliver a coherent maritime strategy. This includes:

expanding our surface combatant fleet

addressing the vulnerability of Australia's limited number of resupply, mine warfare and hydrographic vessels

and resolving longstanding issues around our strategic fleet (commercial ships that could be requisitioned in a time of crisis).

We must also expand our flagged merchant shipping fleet by reforming the Australian International Shipping Register. And we must strengthen our domestic maritime security through the establishment of a national coastguard.

But AUKUS, as the centrepiece of our future undersea capability, is a good start.

AUKUS has attracted plenty of criticism - particularly following the new Trump administration's moves away from the US' traditional allies in Europe.

Yet, despite claims the three-phase AUKUS submarine plan is failing, it remains remarkably on track.

Like any complex defence acquisition, it carries risks. These risks include the continued political will to keep the deal on track, as well as the workforce, delivery schedule and cost pressures that come with building the submarines.

But the relevant question is not whether risks exist - if that were the test, most defence programs wouldn't proceed. The question is whether the risks around AUKUS are being effectively mitigated.

And as the three phases of the AUKUS deal progress, these risks will continue to evolve. Australia must remain focussed on addressing them.

The political risk has been most salient recently, given the Trump administration's actions on Europe, Ukraine, foreign aid and tariffs. But while these disruptions are significant, they were largely foreshadowed.

By contrast, the political signals coming out of Washington around AUKUS have been overwhelmingly positive. This is because AUKUS is in the US' strategic interests as much as it is in Australia's interests.

Importantly, the political commitment to AUKUS in Canberra, Washington and London has already been demonstrated.

The "optimal pathway" to guide the agreement into the 2030s was signed within 18 months of AUKUS' launch in September 2021. And the AUKUS treaty that enables the US and UK to transfer nuclear submarine technology and equipment to Australia has since been signed and entered into force among all three partners.

In Australia, bipartisan support has held for over three years, with no sign of weakening.

Many critics have also focused on the risks posed by the US submarine industrial base and its ability to build nuclear-powered submarines quickly enough.

The US would need to increase its production rate to two Virginia-class submarines per year by 2028 - and subsequently to 2.33 submarines per year - in order to reach the target US fleet of 66 submarines by 2054.

But this does not preclude the sale of three Virginia-class submarines to Australia in the early 2030s. Australia is not just a recipient of submarines from the US - it will help enable the US' undersea operations in the region.

Our role as a rotational hub for US submarines and the longstanding support we can offer the US fleet through facilities such as the Harold E. Holt submarine communications station makes our contribution far more valuable than the notional loss of three submarines on paper.

Could this change in the future? Like all international arrangements, of course it could. But there is no indication at present that it will.

The defence of Australia is not simply about protecting our continent from attack - it is about safeguarding vital national interests. For an island nation, that means securing maritime trade routes and undersea infrastructure.

Even for those concerned about the extremely unlikely prospect of invasion, a robust maritime strategy also enables threats to be defeated well before they reach our shores.

Through its emphasis on maritime power projection, AUKUS reflects a fundamental shift in how we think about defending Australia in the decades ahead.

This is the final part of a series on the future of defence in Australia. Read the other stories here.