The 2018 contingent of more than 1,500 US marines began arriving in Darwin two weeks ago. Every year since 2012 marines have been routinely stationed here, and, under the terms of the force posture agreement between the USA and Australia, they will be here every year until 2040.

This is an extraordinary arrangement, which has come under little public scrutiny. The strategic significance of having the forces of a foreign nation stationed on domestic territory has largely escaped attention. Why this highly unusual deployment has become necessary in peace time has never been properly explained.

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Having foreign troops on home territory creates a potential breach in any sovereign nation’s defence. The first criterion of independence has to be the nation’s capacity to look after itself by conducting its own defence. Under normal circumstances no independent nation entertains the forces of another nation in anything other than the most extreme of situations, such as being under immediate threat from some third party.

In the case of Australia, no such threat exists. The 2016 defence white paper states that there is no more than a remote possibility of a military attack by another country on Australian territory in the foreseeable future. This being the case, there should be some other explanation for such an unusual development. However, successive Australian governments have offered only flimsy reasons.

From the very start, the move lacked transparency. Despite its significance, the decision involved no parliamentary debate and little public discussion. It was simply announced (jointly, by President Obama and then prime minister Julia Gillard) in the Australian parliament in November 2011, as part of the USA’s “pivot to the Asia/Pacific”. It was fait accompli. Whether the idea for the deployment originated in Australia or the USA has never been made clear. The amount that Australia must contribute to the cost of the marines’ presence is secret. The true nature of their mission remains hidden.

Before he died, former prime minister Malcolm Fraser became an outspoken critic of the development. He pointed out that the marines could be used in some offensive operation on orders from Washington – and that Australians would only learn about it after the event. The Australian government has provided no assurance that such an eventuality could not occur.

Several reasons for the deployment have been given in correspondence between peace activists and a succession of defence ministers. It is argued that the marines’ presence will maintain stability in the region – stability underwritten by the USA for the past seven decades. The deployment’s detractors say it is an aggressive and expansionist move, will increase regional tensions, is designed to contain China and will provoke any nation not already aligned with the US.

The government insists that the move is nothing more than an extension of existing arrangements, under Australia’s alliance with the US. It is actually a radical departure from normal practice. In the eyes of some activists, it could ultimately prove to be a step too far, taking Australia to a situation where military commitment to the US becomes detrimental to the national interest.

The government says that the deployment provides greater opportunities for ADF and US forces to train together, and this is true. “Interoperability” has been a key concept in successive defence white papers. However, are interoperability and gaining better training sufficient reasons for hosting foreign forces on a semi-permanent basis?

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Opponents (such as the Independent and Peaceful Australia Network, IPAN) say that, rather than making Australia safer, the deployment provides a target for potential attack.

It is not clear whether the marines are here for Australia’s defence. However, there is a perception, popular among those who recall the second world war, that Australia is always vulnerable to attack, and would have been overrun by Japan were it not for the American armed forces. By implication, this view suggests that Australia is incapable of managing its own defence. This is repudiated by defence expert, Dr Mike Gilligan, who argues that Australia already has adequate capacity to defend itself – giving grounds for ADF personnel to feel offended by the suggestion that Australia cannot manage without the help of the US.

Many questions remain. Which country benefits most from the deployment – Australia from better training and possibly more regional stability, or the US from having another forward position in the region? How are our regional neighbours impacted and how do they view the matter? Above all, what strategic wisdom informed the decision in the first place?