Russia never wanted NATO to spread east through the former soviet republics. But it especially didn't want it to reach Ukraine. A compromise in 2008 put Ukraine on the path to membership, and Russian President Vladimir Putin is now effectively holding the country hostage in effort to keep that from happening.
NPR's Becky Sullivan reports on the history of NATO and how a disagreement over a past proposal is fueling Putin's frustration. Read more about that here.
And NPR's European correspondents describe how U.S. allies France, Germany and the U.K. are attempting to work together to stop Russia from crossing the Ukraine border.