Michael Brune, the Sierra Club’s radical executive director, claimed that “this letter is not in response to what’s happening in Crimea. But clearly it’s a big part of the conversation. The idea that U.S. gas exports could address the human-rights abuses that we’re seeing in Crimea reflects a lack of comprehension and lack of vision.”

Unlike Brune, Putin realizes that energy is a powerful tool. In 2012, just over a third of Europe’s natural-gas imports derived from Russia (most of that travels through Ukraine). The Kremlin-run Gazprom alone accounts for one-fourth of all natural gas used in the European Union. Ukraine itself gets 70 percent of its gas from Russia, and other Eastern Europeans are even more reliant.

In 2009, 2008, and 2006, Putin turned off the taps, cutting natural gas that flows through Ukrainian pipelines and causing major energy disruptions there and beyond. And last fall, as Moldova drew closer to Europe, Russia’s deputy prime minister threatened, “Energy supplies are important in the run-up to winter. I hope you won’t freeze.”