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Shifting from the ‘sea-air gap’ toward ‘forward defence in depth’

As the geo-strategic environment of the Indo-Pacific continues to evolve Australias traditional protective moat, the “sea-air gap” and the strategic advantage it imparts continues to decline, prompting the Australian government and strategic policymakers to consider a new doctrine of “forward defence in depth”, writes Dr Malcolm Davis of the Australian Strategic Policy Institute.

Forward defence in depth for Australia, a new report by ASPI’s Defence Strategy and Capability section, advocates that in the next defence white paper, Australia should update its traditional military strategy that for decades has focused on the defence of the ‘sea-air gap’ between Australia’s northern coastline and Indonesia.


The ASPI report advocates a new strategy – ‘Forward Defence in Depth’ – which argues that the ‘sea-air gap’ shouldn’t be viewed as the front line but become the main rear area and suggests ADF air and naval forces be projected deep into maritime south-east Asia, and the south-west Pacific to meet a threat from a major adversary as far away from Australia as possible.

The new strategy is necessary to meet the growing challenge posed by China’s increasingly powerful military capability, notably, its advanced ‘anti-access and area denial’ (A2/AD) capabilities, which are increasingly able to threaten Australia’s north. It also allows Australia to burden share to a greater degree with the United States in line with US desires for greater allied contribution as noted in the 2018 National Defense Strategy.

Author Dr Malcolm Davis said that “the traditional focus on the ‘sea-air gap’ as a strategic moat that can be defended is now more akin to the 21st Century Maginot Line mindset. Modern long-range strike weapons now appearing in China’s PLA will increasingly erode the viability of any strategy that envisages an ADF sitting behind the sea-air gap.

The ADF must instead adopt a new strategy of forward defence in depth, which would see air and naval forces project power deep into maritime south-east Asia and the south-west Pacific, supported by enhanced defence diplomacy that builds greater forward access and partnerships with key allies,” he said.



The new strategy would also require new force structure beyond that planned for in the 2016 Defence White Paper, specifically, greater capability for rapid long-range power projection.

The report found that:

  • China’s development of long-range ballistic and land-attack cruise missile capabilities are expanding its ability to strike at base infrastructure across northern Australia, and development of hypersonic weapons will increase that risk;
  • China’s evolving air and naval capabilities are increasing its ability to project power into Australia’s air and maritime approaches; and
  • China’s counter-space and cyber space capability against Australia circumvents the sea-air gap entirely.

The report argues that a new strategy of forward defence in depth would respond to this challenge directly and would see Defence policy adapt to a deteriorating strategic outlook. Key suggestions include:

  • Enhanced defence diplomacy, focusing on building closer ties with Indonesia, and other south-east Asian states, as well as in the south-west Pacific.
  • Establishing a formalised trilateral defence alliance between Japan, the US and Australia, including reciprocal basing along a ‘Okinawa-Guam-Lombrum-Tindal’ defence chain; and
  • In terms of force structure, the ADF should acquire new power projection capabilities for the ADF including long-range air power; place increasing emphasis on manned-unmanned teaming; acquire sea and land-based BMD to counter Chinese conventional ballistic missiles; develop enhanced C4ISR capabilities including sovereign space capabilities; increase investment towards operationally deployed hypersonic weapons; and accept greater dependence on unmanned systems in the air, in ‘near space’ as well as on and under the sea, including large UUVs to complement Collins and Attack Class future submarines.

With the return of the Morrison-led Coalition government, the report argues that now is the time for a fundamental review of ADF military strategy, ideally as part of a new Defence White Paper process that would revisit assumptions on strategy, force posture, force structure and defence spending. 

Dr Malcolm Davis joined ASPI as a senior analyst in defence strategy and capability in January 2016 and has expertise in a number of areas, including strategy and capability development, future warfare and military technology, Chinese military modernisation and Asian security.

The full ASPI report by Dr Davis is available here

Shifting from the ‘sea-air gap’ toward ‘forward defence in depth’
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