Several readers have written in to ask, in so many words, whether Russia will invade Ukraine.  The short answer is that we still don’t know. And we don’t know if Vladimir Putin knows. What we do know is that the December 7 virtual summit between him and Joe Biden did nothing obvious to diffuse tensions on the Ukrainian border. Not only has Putin not recalled his troops in the week since his conversation with the American president, videos and other reports trickling in from the Russian side of the border paint a worrying picture: rows upon rows of tanks and heavy artillery stretching as far as the eye can see. Russian troops are moving, but not in a way that would alleviate some of the pressure they’ve created. (One Washington diplomatic source told me the troop movements were “unhelpful” and “worrying.”)