SYDNEY —Australia unveiled AU$3.9 billion (US$2.8 billion) in spending on Sunday as a “down payment” on a new facility to build nuclear submarines under the tripartite Aukus security pact with Britain and the United States. The Aukus pact aims to arm Australia with a fleet of cutting-edge submarines from the United States and would provide for cooperation in developing an array of warfare technologies.The submarines, the sale of which will begin in 2032, lie at the heart of Australia’s strategy of improving its long-range strike capabilities in the Pacific, particularly against China. The deal could cost Canberra up to US$235...