The South China Sea is one of the world’s most contested areas, where countries have been busy transforming tiny reefs, rocks, and submerged features into militarized islands. More than 90 outposts have been built across about 70 locations, according to satellite images studied by the Asia Maritime Transparency Initiative. But why is this patch of sea so important, and what’s fueling this frenzy?

Why the South China Sea Matters

Beneath the waves lie vast reserves of oil and gas, essen...