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Vladimir Putin’s inauguration to a third term as Russia’s president today, after a first-round victory in March elections, shows he is both shrewd and skillful. He is also very lucky, in a way that should worry us.

Putin’s great luck comes from his timing. The fate of the Russian economy -- and the popularity of its leaders -- is closely tied to the price of petroleum. Oil and gas account for almost two-thirds of Russia’s exports, and about half of the federal government’s revenue.

Putin first took office as then President Boris Yeltsin’s prime minister in August 1999, when global oil prices had just passed their lowest point since the early 1970s. During his first two terms as president, global oil and gas prices more than tripled. Thanks to a flood of...