The escalating crisis also raises questions about whether the White House was quick enough to recognize the seriousness of the Ukraine issue and to give it adequate attention.
U.S. officials and other sources said that the State Department, particularly the hard-charging assistant secretary of state for Europe, Victoria Nuland, had for months been raising alarms about Russia's more aggressive posture toward former Soviet states, and Ukraine in particular.
Washington's engagement accelerated after a November 2013 European summit at which Ukraine - along with Armenia - declined under heavy Russian pressure to sign association agreements with the European Union.
"That's when you saw the Americans stepping up," said Damon Wilson, executive vice president of the Washington-based Atlantic Council, and a former adviser on Europe to President George W. Bush.
Nuland, he said, "created U.S. policy really out of very little at the time."