Barack Obama warns progressives to avoid 'circular firing squad'

* Remarks come as Democrats battle for 2020 nomination* Former president was addressing young Europeans in GermanyBarack Obama addresses a town hall of young leaders from across Europe at an Obama Foundation event in Berlin, Germany, on Saturday. Photograph: Fabrizio Bensch/ReutersBarack Obama warned on Saturday that US progressives risk creating a “circular firing squad” at a time when prospective presidential candidates are competing fiercely against each other to run against Donal