A generation ago, Jared Diamond argued in his bestselling book Guns, Germs, and Steel: The Fates of Human Societies that the most important factor in explaining why one society flourishes and another deteriorates is the accident of geography. Natural resources and climate, not political culture or worldview, determine the success or failure of civilizations. For a great many learned people in Europe and North America, it was a seductive thesis. But the present state of two South American neighbors—Colombia and Venezuela—makes it awfully hard to take Diamond’s view seriously.

On Tuesday Colombia inaugurated a new president, Iván Duque, a young conservative who won office in June by promising pro-growth policies and vowing to take a tougher stance against gangs and drug cartels.