t is a puzzle at the heart of the crisis over Russia’s threat to invade Ukraine: Why has President Biden, more than one year into his presidency, failed to name an ambassador to Kyiv?

Neither the Biden administration nor Ukraine’s government is providing a clear explanation for a delay that career diplomats say would be baffling and inexcusable even in ordinary times, never mind at a moment when the U.S. relationship with Ukraine is as consequential as it has ever been.

Experts say that the presence of a full-time ambassador could help to smooth awkward relations that have emerged between the Biden administration and the government of President Volodymyr Zelensky despite Ukraine’s heavy reliance on Washington for its defense against Russia. But it is also unclear how eager the Ukrainians are to receive an envoy from Mr. Biden, who submitted a candidate to Kyiv for approval weeks ago.