Another Must Read from Krugman

He's talking about free trade and transportation costs, and makes the point that when the costs of transporting goods falls relative to manufacturing goods and services, as they did in the 1870-1913 era, international trade increases, and when the costs of manufacturing goods falls relative to transporting goods, as they did in the era of Henry Ford's assembly line, pre containerized cargo 1913-1970 or so, international trade falls.

Go read, it's 599 words well spent.