The deployment was a hallmark of the Obama administration's "pivot" away from the Middle East to Asia. "We are here to stay," the President told a joint sitting of the Australian Parliament after announcing the Marine decision.

The permanent rotating deployment of up to 2500 Marines was announced jointly by the Prime Minister, Julia Gillard, and the US President, Barack Obama, as planned, on November 16 last year. Australia's tremulousness struck between September and November.

One reason for the US surprise and concern was that Canberra first proposed the idea of US troop deployments in 2010.

A US participant said it was a case of "cold feet". "As you get closer, you realise the momentous nature of it."

The winner of the US presidential election on Tuesday will make decisions on the second phase of the new intensification of the Australian alliance, which will be decided in 2016.

The US did not explicitly threaten to cancel Mr Obama's visit but a US official said it was made clear that "the President was not going to Australia to announce some temporary measure or 250 Marines. It was pretty clear that it would not be good for anyone for the President to have that sort of visit. This was very important to the White House."

A separate illustration of Australian political nerves arose when a US official suggested the Pentagon send 7500 Marines to Australia, three times the number ultimately agreed. Deployments were discussed in multiples of 2500 because this is the minimum size of an independent Marine fighting force, defence officials said.

This idea was eventually killed off by both sides, officials said. The Gillard government was concerned it would look "too big" politically, that the government would be seen to be conceding too much to the US.

And the US Defence Department dismissed the idea as operationally undesirable: "It was too many in one place. We'll have four sets of these forces, two in Japan, one in Guam, and one in Australia. It's a better dispersal than having three in Australia."