How should the arrival of US marines in the Australian northern territories be interpreted by China?

Well, two hundred arrived in Darwin today, with the number to be bolstered to 2, 500.

This presence will mean that the US will be better prepared for any engagement in the South China Sea, which is a major commercial shipping route and which has also been the focus of dispute with China, who regards this as her own territorial waters.

America also has marines stationed in Central Asia, Iraq and Afghanistan.

Trouble is no one seems to be asking why it is doing all this.