The expansion will include a dock for a nuclear-powered aircraft carrier, a missile defence system, live-fire training sites and the expansion of the island's airbase. It will be the largest investment in a military base in the western Pacific since the Second World War, and the biggest spend on naval infrastructure in decades.
However, Guam residents fear the build-up could hurt their ecosystem and tourism-dependent economy.
Estimates suggest that the island's population will rise by almost 50 per cent from its current 173,000 at the peak of construction. It will eventually house 19,000 Marines who will be relocated from the Japanese island of Okinawa, where the US force has become unpopular.
The US's Environmental Protection Agency (EPA) has said that this could trigger serious water shortages. The EPA said that dredging the harbour to allow an aircraft carrier to berth would damage 71 acres of pristine coral reefs.
The EPA's report said the build-up would "exacerbate existing substandard environmental conditions on Guam"....