Europe threw its support on Friday behind a special tribunal to prosecute President Vladimir Putin and other senior Russian officials for the crime of aggression against Ukraine, a show of unity on a day when Moscow marks its victory day.

“The crime of aggression is sometimes referred to as the ‘mother of all other crimes’ because it precedes all of the other crimes, war crimes, crimes against humanity, even genocide,” Iva Vukusic, an international law expert at the University of Utrecht, told The Associated Press.

Ukraine and Hungary said on Friday they were expelling two diplomats each after each side accused the other of engaging in espionage.

Ukrainian security services claimed to have uncovered a Hungarian spy ring gathering information about military activities in Ukraine’s western Transcarpathia region.