Erich Priebke was the SS officer responsible for the execution of 335 Italian hostages in the Fosse Ardeatine. He was later charged with five murders. Three hundred and thirty hostages were lawfully shot in retaliation for an attack on German soldiers in which thirty-three had died; Hitler ordered a 10:1 ratio for retaliation. Until 1949, and the IVth version of the Geneva Convention, the shooting of hostages in certain circumstances by occupying armies was perfectly legal. An invaded civilian population was expected to be compliant to the rule of the invaders; the convention allowed the collective punishment of the population for infractions. And so Priebke was guilty only for the five additional victims of the Ardeatine massacre that he added himself.
The 1949 Geneva convention that the UK signed up to (but the US didn't) made any future Dambusters crews war criminals (attacking dams a war crime under Articles 56 and 53 of Protocol 1) and indeed made 'Bomber' Harris' entire city bombing campaign illegal for the future, and every member of the RAF's WWII bomber crews potential war criminals (Articles 51 and 54; attacks on civilian targets). It also gave the rights of combatant soldiers to guerillas - including the right to protection if rendered hors de combat on the battlefield.
Of course, before the 20th century the individual lives of war's victims, civilian or military, were accorded far less importance. There's a fine piece in the Indie this morning by John Lichfield that catalogues how the CWGC changed everything after the Great War; the dead of Waterloo were used to provide replacement teeth, and after to fertilise the fields of the Midlands. Today the footways of Royal Wootton Bassett are five-deep in respectful mourners as each flag-draped service coffin passes through.
Following the debate on the issues surrounding the Ardeatine massacre - were the killers of the German soldiers terrorists or freedom fighters? - István Deák wrote that although "armed resistance during
World War II was romanticized because the Nazis were such an appalling
enemy, and because in that war the guerrillas' targets were still mainly
soldiers", it is increasingly hard to draw the line between freedom
fighting and terrorism. In his opinion, the Hague Conventions regulating
irregular warfare has been "more a failure than success." "What is
needed," Deák stresses, "is a recognition of reality, namely that future
wars will increasingly consist of civilians shooting at soldiers from
hiding and frightened soldiers killing innocent civilians. And what is
needed, in the aftermath of such a sobering recognition, is an attempt
to create a new international law for the more efficient regulation of
this type of horrible warfare."