“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.

The visit to Ukraine will be the first for Mr. Merz as German chancellor, and is the first time leaders of four European nations have traveled together to Ukrainian soil.