It would be hard to image a more fitting symbol of Russia’s declining military fortunes than the sight of a solitary Stalin-era tank trundling across Red Square during the country’s traditional Victory Day celebrations on May 9. For the past two decades, Vladimir Putin has used Victory Day to showcase modern Russia’s resurgence as a military superpower, with dozens of the very latest tanks typically taking part in each annual parade. This year, however, the only tank on display was a T-34 model dating back to World War II. Inevitably, the embarrassing absence of tanks at this year’s Victory Day...