In an apparent response to Donald Trump hinting that his administration might allow Ukraine to “fight back” by attacking Russia itself, Vladimir Putin made a surprise visit Friday to Sarov. The city, located in Russia’s Nizhny Novgorod region, has long been associated with nuclear weapons research. The escalating rhetoric from the U.S. and Russian presidents comes just a week after the two leaders met in Alaska and appeared to agree on the broad contours of a peace settlement in Ukraine.