Russian opposition leader Alexei Navalny returned to Moscow, following a failed poisoning attempt by Kremlin security services. He was arrested upon arrival and subsequently sentenced to two and a half years in prison for allegedly violating parole. As Russians take to the streets to protest the Kremlin's show trial and international calls for Navalny’s release grow, our experts react to what may be the most serious challenge to Vladimir Putin’s rule in 20 years.