Russian President Vladimir Putin has spelled out a nationalist rationale for his country’s military incursion into two restive provinces in eastern Ukraine largely controlled by Kremlin-backed separatists, but it is primarily about protecting Moscow’s energy interests. That was true in 2014, when Russia seized Crimea and I was a Moscow correspondent for the Wall Street Journal, for which I wrote dozens of stories about the insurgency in Donetsk and Luhansk that Russia helped foment. And it remains true now. To understand the Kremlin’s motivations in regard to its smaller, and relatively impoverished, neighbor, the key fact to know is that...