American-Vietnam War theater
Da Nang was a prime target of activity during the American-Vietnamese war. The city of Hoi An was protected by decisive action of the warring parties.
Marble Mountain's caverns were used as various depots and even as a hospital ward since it is naturally hidden and protected.
Also as a side trivia point, the USA only lost five C-141's. One of them was in DaNang. See if anyone is selling used airplane parts. I didn't see any parts of the C-141, but it will give you something to do while walking down the busy streets with nothing much to visit.