DARWIN, Australia —The past casts a long shadow over this palm-lined city on Asia’s fringe. Allies upgrade facilities and Marines expand operations as China extends its reach. Darwin’s location, 400 miles from the Indonesian archipelago and 1,700 miles from the South China Sea, provides what could serve as a springboard for U.S. forces in the region.

For the U.S., Australia’s benefits go beyond confronting China. The alliance allows the U.S. to disperse forces and offset a lack of permanent bases in the Pacific outside Japan and South Korea, particularly for aircraft.