By Jacqueline Thomsen and Mike Scarcella (Reuters) – U.S. President Joe Biden on Monday called for the prosecution of Russian President Vladimir Putin for war crimes over the discovery in Bucha, Ukraine, of mass graves and bodies of bound civilians shot at close range, but various challenges stand in the way. IS TARGETING CIVILIANS A WAR CRIME? Yes. The International Criminal Court in The Hague defines war crimes as “grave breaches” of the post-World War Two Geneva Conventions, which lay out humanitarian laws to be followed in war time. Attacking legitimate military targets where civilian casu…