The Justice Department's surprising decision to drop all charges against Donald Trump's former national security adviser has set off a cascade of accusations and counter-accusations. And at the centre of it are the present and previous presidents. Michael Flynn was forced to resign early in the Trump presidency for lying to Vice-President Mike Pence about his contacts with Russian government officials. He had also pleaded guilty to lying to FBI agents investigating ties between Russia and the Trump campaign. After years of legal wrangling, however, the former three-star general is now a free man. What are Trump and Obama saying? Obama has largely held his tongue about the actions of his successor but on Friday in a call to 3,000 former aides and officials that was leaked to the media, he let loose. "There is no precedent that anybody can find for someone who has been charged with perjury just getting off scot-free," he said. "That's the kind of stuff where you begin to get worried that basic - not just institutional norms - but our basic understanding of rule of law is at risk." Trump, who has never been shy about criticising - or blaming - his predecessor, responded with a flurry of social media posts and retweets on Sunday, accusing Obama and his aides of engaging in a criminal effort to undermine his presidency. He amplified the assertions of right-wing commentators and lashed out at Obama, former Deputy FBI Director Andrew McCabe, former Deputy Attorney General Rod Rosenstein, late-night television host Jimmy Kimmel and numerous reporters and media outlets. "The biggest political crime in American history, by far!" the president wrote, retweeting a conservative talk-radio host who accused Obama officials of sabotaging Trump in the days before he took office.