President Biden said Wednesday that he believes Russian President Vladimir Putin will invade Ukraine: “My guess is he will move in. He has to do something.” Now all eyes are on Volodymyr Zelensky, the 43-year-old president of Ukraine, a country menaced by 100,000 well-armed Russian troops. Recent U.S.-Russia-NATO talks failed to resolve this tense situation. Instead, Russia maintained its extreme demands, insisting that NATO cease its expansion and that Ukraine be barred from joining the alliance. Neither Washington nor NATO agreed to these terms, but Biden has ruled out sending U.S. forces to Ukraine. Washington had threatened sanctions over an invasion, but Wednesday, Biden wavered, saying a “minor incursion” might not lead to harsh sanctions after all. That means if Russia invades, Ukraine could be on its own. Biden, attempting to clarify the U.S. position, said Thursday that if “any assembled Russian units move across the Ukrainian border,” that would be an invasion, which would be met by a “severe and coordinated economic response.” Also Thursday, Zelensky tweeted: “We want to remind the great powers that there are no minor incursions and small nations. Just as there are no minor casualties and little grief from the loss of loved ones.” This year marks the 100th anniversary of the founding of the Soviet Union, and some experts suggest that Putin may be aiming to restore some of its lost territory, starting with parts of Ukraine. Zelensky took time earlier Wednesday, before Biden’s news conference, to speak to The Washington Post’s Lally Weymouth by videoconference. Edited excerpts follow: