Experts on the subject are divided into two groups: those who think sanctions usually fail and those who think they almost always fail. Gary Hufbauer, an economist at the Peterson Institute for International Economics in Washington, says sanctions have been effective in about 30 percent of the cases they’ve been used. But he doubts the steps taken by Obama — what he calls “light” sanctions — will make any difference in Ukraine.

“The success rates for symbolic or ‘light’ sanctions, for sanctions against autocratic governments, and for sanctions seeking territorial concessions are lower,” he said by email. For anyone hoping to get the Russians out of Crimea, he said, “these findings are not auspicious.”

Pessimists are even gloomier. University of Chicago political scientist Robert Pape calculates that sanctions have worked less than 5 percent of the time. The intractable obstacle, he has written, is that modern governments are “willing to endure considerable punishment rather than abandon what are seen as the interests of the nation.”

Crimea is unquestionably regarded as a vital interest by Moscow. Weak regimes are more susceptible to pressure, but Vladimir Putin’s regime is not a weak one.