On the eve of the new president's inauguration, Panorama investigates Donald Trump's strange bromance with Vladimir Putin. John Sweeney - who has confronted both men in the past - travels to Russia, the United States and the battlefields in Ukraine to report on what's behind their mutual admiration. He investigates whether Russian cyber-warriors helped get Donald Trump into the White House and asks how safe the world will be if they stay friends - or if the bromance falls apart.