Last week, a man talking like a sheriff rode briefly into an Eastern European town to present folks there with what sounded a bit like a promise: If Vladimir Putin pressed his increasingly successful policy of limited sovereignty for Russia's neighbors on the 3.2 million fine people of Moldova, he would have to answer to America.

John Kerry, the man in the notional white hat in Chisinau, the Moldovan capital, told President Nicolae Timofti before galloping off: "I am here to affirm to you that the United States will...