A bureaucrat burned China's proof that it once ruled the seas
Ming China sent seven fleets to Africa, then a single official erased the record so no one would try again.
Between 1405 and 1433, the Ming dynasty sent Admiral Zheng He on seven voyages that reached the Persian Gulf and the coast of East Africa, decades before Portugal's caravels went nearly as far. The ships were reportedly enormous by the standards of the age, and the fleets carried thousands of sailors, soldiers, and diplomats bearing silk and porcelain to trade for ivory, spices, and even giraffes.
Then, within a generation, the voyages simply stopped. The Xuande Emperor's successors turned inward, worried about the cost and about a court faction that saw the sea as a distraction from the real threats on China's northern border.
The fleets might have been merely defunded and forgotten, except for what happened next. Later in the century, an official named Liu Daxia was asked to retrieve the naval archives so a new expedition could be planned. He hid the records instead, then destroyed them, dismissing the old voyages as "deceitful exaggerations of bizarre things" unworthy of a serious court's attention.
His reasoning has outlived him by five centuries as a case study: it's rarely the failure of a bold undertaking that erases it from memory. It's the paperwork clerk who decides, quietly, that the file shouldn't exist.