Obama ... is scheduled to make the announcement with Julia Gillard when they visit Darwin next week. Credit:AFP ''This is all about the rise of China, the modernisation of the People's Liberation Army and, particularly, it's about the increased vulnerability of US forces in Japan and Guam to the new generation of Chinese missiles,'' said Alan Dupont, the Michael Hintze professor of international security at Sydney University. ''The new Chinese missiles could threaten them in a way they've never been able to before, so the US is starting to reposition them to make them less vulnerable. Australia's 'tyranny of distance' is now a distinct strategic advantage.'' Professor Dupont, a former Australian Defence official and intelligence analyst, said the ''Australian strategic rationale is that we are also hedging against increasing Chinese military power and their capacity to destabilise maritime trade routes. And we want to get closer to the US. ''There's no doubt at all the Chinese will have serious reservations about this''

Mr Obama and Ms Gillard are not expected to argue that China is a factor in the decision. ''This is a strong gesture that even in the face of budget constraints, the US reaction to the winding down of the wars in Iraq and Afghanistan deployment is not to go home but to pivot'' into the Asia-Pacific, according to the former deputy secretary of state in the Obama administration, Jim Steinberg. But Hugh White, a professor of strategic studies at the Australian National University and a former deputy secretary of defence, said the decision would have deep consequences for Australia's relations with China. ''I think this is a very significant and potentially very risky move for Australia. In the view from Beijing, everything the US is doing in the western Pacific is designed to bolster resistance to the Chinese challenge to US primacy. ''In Washington and in Beijing, this will be seen as Australia aligning itself with an American strategy to contain China.'' Mr Obama and Ms Gillard are to say the US will not build a new base for the Marines but will use the Robertson Barracks, the Australian base near Darwin.

But the base is home to about 4500 Australian soldiers and has capacity for only a couple of hundred more. The facilities will need to be expanded to accommodate the US Marines on rotation, whose numbers are expected to build. Such a decision has been under consideration for some years. The Marines are to use the base for training. ''They want to be able to fly helicopters, drop out of planes and shoot at things, and you can't do that in crowded Okinawa,'' in the words of Mike Green, a former top Asia adviser in the George W. Bush administration. The Greens oppose any expansion of the US military presence in Australia. By using an existing Australian base rather than building a new US one, the Pentagon considers the new presence will be more ''politically sustainable''. The then US defence secretary, Robert Gates, said last November in Melbourne: ''We don't want to do things that would be politically difficult for the Australian government. We want to enhance the alliance, not create controversy.''