The ratio of world trade to output was a mere 2% in 1800, but it then rose to 10% in 1870 to 17% in 1900 and 21% in 1913. It then fell back to 14% in 1929 and only 9% in 1938. The period 1870-1913 thus marks the birth of the first great era of trade globalization, the period 1914-39 its death. What caused the trade boom and bust? The textbook interpretations offer a variety of narratives, but few precise answers. We use an augmented gravity model of trade to examine the gold standard, tariffs, and transport costs as determinants of trade. In the nineteenth century the gold standard was much more important than tariff policy, and just as important as transport costs as a trade-creating force. In the 1920s, the slowdown in trade was driven by a rise in transport costs, though trade barriers other than tariffs might have been important. In the 1930s, the final collapse of the gold standard, persistently high transport costs, and the expansion of other barriers drove trade volumes even lower.