On 15 May, the day I arrived in Australia to begin work at the Lowy Institute, Prime Minister Tony Abbott was dousing a media bushfire. The spark was the apparent revelation by US Assistant Secretary of Defense David Shear that US B-1 bombers and surveillance aircraft were to be based on Australian soil; Shear had apparently framed this decision around China's behaviour in the South China Sea.

Mr Shear's 'mis-spoken' testimony to a Senate Committee was quickly retracted and the written record contains no reference to specific US aircraft in Australia, only to 'enhanced access for US forces'.

That the Prime Minister felt compelled to intervene on such a detail struck me then as an interesting straw in the wind. Yet, delivered from the incongruous surrounds of a pre-school sandpit in Sydney, it hardly felt like an inflection point in the ANZUS alliance. Now I'm beginning to wonder, at least when it comes to the public debate.

Within three short weeks of that fleeting controversy, as Australian policymakers have mulled over whether to support the US in asserting freedom of navigation in the South China Sea, a veritable sceptics chorus (eg. Sam Bateman, Geoff Miller and Hugh White) has cautioned from the sidelines against active Australian involvement there.

This 'heated' public discourse has not gone unnoticed by Washington think tanks and reveals how quickly polarised the debate has become here as the US-China strategic dynamic weighs more specifically over the defence and security policy choices confronting Canberra.

I still think that framing the US-China relationship as an exclusive choice, while a useful proposition for stirring debate within Australia, presents a false binary. In regard to the South China Sea, I've argued in the Australian Financial Review that Canberra can and should aim to chart a course between avoiding tokenism within the alliance without being unduly provocative to China.

Cleaving a distinctively Australian middle road through these tricky Southeast Asian shoals could be enabled by the Five Power Defence Arrangements, a useful sub-treaty multilateral framework, though one so low-key and testosterone-lite that it is sometimes at risk of being forgotten altogether.

Nonetheless, both the B-1 bomber media furore and the ongoing 'freedom of navigation' debate in the South China Sea have between them demonstrated that Australia is now entering an era of more difficult choices, buffeted by the increasingly inclement Sino-US strategic weather. Opinion on the alliance with the US is arguably starting to divide more significantly than at any time since the 2003 invasion of Iraq.

If first impressions are anything to go by, beyond the threats posed by terrorism and radicalisation, Australia's debate on defence and security is still mainly an elite affair. However, that is likely to change as the challenges of US-China strategic competition become less abstract and more concrete. As this happens, establishing a viable centre ground in the debate around alliance issues will become more important, politically. In the dawning era of more awkward alliance choices, that middle ground is beginning to look a little uncertain.

Photo by Flickr user Dean Hochman.