Four years after Russia clambered on to the sunlit uplands of investment-grade territory, it is sliding back down the slope into junk territory. Standard & Poor’s downgrade of Russia’s foreign currency rating to BBB leaves it just two notches inside investment grade.

While markets largely shrugged off the move, it shines an uncomfortable spotlight on the government’s choice to depreciate the rouble in a series of mini-steps rather than in one big move. The decision is forcing it to burn reserves alarmingly fast to defend the currency, under heavy pressure from plummeting oil prices. Arguably the government had little choice, since the revived rouble is a symbol of Russia’s renaissance. But the policy now looks like a gamble on Moscow being able to control the situation by sporadically widening the rouble’s trading “corridor” against a dollar/euro basket, until a rising oil price comes to the rescue.