Since the first world’s fair in 1851, held in the groundbreaking Crystal Palace in London, the exposition universelle has been a driver of experimental architecture and engineering. Often, these monuments—such as the Eiffel Tower (which was intended to be temporary), Ludwig Mies van der Rohe’s Barcelona Pavilion and the Space Needle in Seattle—live on long after the fair has been forgotten, having given the architects the rare freedom to express themselves without many of the usual functional requirements associated with large-scale commissions.

For Expo ’98 in Lisbon, the first world’s fair to be held in Portugal, the organizers approached Álvaro Siza, one of the country’s most acclaimed architects, to design the national pavilion and centerpiece of the festival. He work...