On July 16, 1938, Franklin Delano Roosevelt boarded the USS Houston in San Diego and embarked on what would come to be known as the Third Presidential Cruise. Publicized as a presidential fishing trip, the 1938 excursion was considerably more than a summer vacation. Covering 5,888 miles in twenty-four days, it comprised fourteen stops in the territories of five different countries: Mexico, France, Ecuador, Costa Rica, and Colombia. Concluding in Pensacola, Florida, after passing through the Panama Canal, it was an (...) ambitious voyage that involved the intensive use of science as diplomacy, a mere thirteen months before the onset of World War II. On the surface, the cruise, whose collecting activities were supervised by the Smithsonian’s Division of Marine Invertebrates, was a major scientific coup. However, the expedition also had a clear geopolitical goal: to assess the United States’ naval capabilities in the Pacific and to formulate potential military strategies in the event of global war. Thus, the excursion illustrates how science and diplomacy can intersect at pivotal historical moments, and how it can be deployed to implement foreign policy based on military interests. (shrink)