Which crop made Vietnam the world number two almost from nothing?

Coffee. Vietnam produced a negligible amount in 1990 and is now the second largest producer on Earth after Brazil, and by far the largest producer of robusta - the harder, more bitter, more caffeinated species used in instant coffee and in espresso blends.

It was a deliberate state programme following the doi moi economic reforms, with land redistribution, subsidised credit and large-scale resettlement into the central highlands. Production increased something like twentyfold in twenty years, which flooded the global market and contributed to the coffee price collapse of the early 2000s - a crisis that devastated smallholder growers in Central America and East Africa. One country's successful development policy was another region's rural catastrophe.

The domestic coffee culture that came with it is its own thing. Vietnamese coffee is traditionally brewed strong through a small metal filter over sweetened condensed milk - a habit that began because fresh milk was scarce under French colonial rule - and Hanoi has egg coffee, invented during a milk shortage in the 1940s by whipping egg yolk with sugar instead.