By Guy Faulconbridge and Andrew Osborn LONDON (Reuters) – President Vladimir Putin’s latest warning that he is ready to use nuclear weapons to defend Russia amid the war in Ukraine has made a troubling question much more urgent: Is the former KGB spy bluffing? Putin cautioned it was no bluff, and Western politicians, diplomats and nuclear weapons experts are divided. Some say he could use one or more smaller, tactical nuclear weapons to try to stave off military defeat, protect his presidency, scare off the West or intimidate Kyiv into capitulation. Putin’s warning, which was followed by a mor…