The Bryce Report was an attempt to verify atrocity stories especially about the attacks against civilians by Germans invading Belgium. These stories had been in wide circulation, and there had been considerable skepticism about them. Bryce himself, who was a widely respected historian and diplomat, reported himself as skeptical.
The information was gathered from a number of sources: refugees living in areas held by the French and British, including refugees in Britain (1200 witnesses in all); official accounts published by the Belgian government prior to the fall of their government; and accounts in diaries captured from German soldiers. The report was published in two volumes: one includes a summary of the kinds of atrocities which the Report takes as credible; the other are a selection of accounts themselves (500 statements from witnesses; excerpts from 37 German diaries).
The reports of the atrocities themselves would not have been news to the general public. This includes the reports of a pattern of systemmatic actions against civilians undertaken by soldiers, including those acting under orders from their superiors, as well as systemmatic attacks against cultural sites, such as the razing of the library at Louvain.
What is important about the Bryce Report is (a) it legitimates the accounts of attacks against civilians and cultural sites and (b) it contextualizes them as proving violations of the conventions governing warfare (which Germany had signed) and as crimes against humanity. The Bryce Report is important as a formal indictment of the Terror Campaign of the Germans.
There is no doubt that the Germans used a terror campaign against civilians as a counter to what they claimed was the unlawful use of snipers against their forces. The German government published a number of responses both during the war and afterwards to legitimate actions against civilians as within the allowed boundaries of the conventions of warfare; i.e., the German White Book(1915) does not deny rounding up the men in Dinant and shooting them, which is claimed as legitimate retaliation for unorganized resistance; it denies that the killing of the wives and children who clustered around the men was part of the plan -- that they simply got in the way having "left their station which was apart from the male hostages." A report published by the German Reichstag in 1924 justified the terror campaign on the grounds that ruthlessness shortens war: "In war everyone is a sacrificial lamb. The humane thing is, in a higher sense, often the most inhumane." By terrorizing the citizenry, the Germans sought to stop their resistance to being occupied -- which is what made the "terror" "humane."
In addition to indicting the terror campaign, the Bryce Report also notes that conducting such a campaign will unleash predictable forces within troops who should otherwise be submitted to strict discipline; this, the Bryce report argues, is what happened in Belgium, particularly with regard to rape.
The report is very hesitant to assert that there was systemmatic rape of women ordered by officers even when there are eye-witness reports, and acknowledges that men committing rape were evidently subject to punishment by superior officers; however, the point the Bryce report makes is that the officers clearly and consistently lost control of their men, and that this is hardly surprising given an official terror campaign against civilians.
Both, then, are part of the indictment against the officers and their masters: ordering troops to committe a systemmatic terror campaigns against civilians, and regular failure by officers to control their troops.
The Bryce Report was used for propaganda purposes. Sir Gilbert Parker, who was the member of Wellington House (the British propaganda bureau at that time) charged with information and propaganda aimed at the United States, rushed the Bryce Report into print, so it was available five days after the news of the sinking of the Lusitania. It is obvious that part of the aim was to contribute to the effort to bring the United States into the war.
At about the same time, the French published a report with was translated into English and published under the title Germany's Violation of the Laws of War. This account offers little beyond a catalogue in formal, detached bureaucratic prose of the actions of the Germans against French populations in occupied territories. The evidence presented is that of French officials, refugees, and also from captured diaries. Unlike the Bryce report, it does not offer a fully developed analysis to support the indictment of a systemmatic campaign of terror and of a failure of officers to control their men with regard to rape. There are apparently two reasons for the sparse style of the French report. One is the awareness of the skepticism that Germany, which was a civilized nation, would engage in atrocities. The effort was to present the evidence in as disinterested or objective style as possible, to let the reader be the judge. The other was the unwillingness on the part of the French government to inflame the imaginations and reactions of those French living in French territorities who had friends and relatives in the occupied territories.
The post-war revisionism had the effect of discrediting not simply the excesses of propaganda that appropriated the atrocity stories, not of sifting out false stories (such as the one that Germans were sending their dead soldiers to be made into soap), but of discrediting the Belgian refugees as eyewitnesses (nothing but a bunch of drunken layabouts, according to one post-war revisionist). This has had the effect of discrediting all stories about the German campaign against civilians and cultural sites; i.e., all "atrocity" stories are tarred with the brush of "mere propaganda" and hence all are "lies" or not "verifiable."
The other dominant approaches have been equally useless in assessing the German occupation of Belgium; i.e., the claim that the Germans acted no differently than other countries (with no evidence of this offered) or that after all, war is hell (which masks the shift in the understanding that civilians are, under that claim, legitimate targets during a war).
These discrediting approaches have had long-lasting effects, even though they were originally undertaken in the United States primarily to serve the interests of Isolationists prior to WWII. They sought to discredit stories of German atrocities reported from the European theatre. The claim was that the United States had entered World War I because of an effective British propaganda campaign directed at the United States. Even though it proved sadly naive to portray the WWII stories as merely British propaganda (and hence "lies"), the effort to debunk the WWII stories by debunking WWI atrocity stories as nothing more than British propaganda seems to have stuck.
This is unfortunate. For one thing, it limits the ability to grasp the steady shift from a morality that condemned warfare directed against civilians as barbaric to one that understood civilians as contributing to the war efforts and therefore as reasonable targets; hence, the bombing of cities, for example. It could also be argued that the belief, promulagated by the reaction against British WWI propaganda, that any atrocity story is automatically propaganda, has tended to blunt our ability to react to reports of modern warfare. One might think, for instance, of the recent general unwillingness to respond in a psychological as well as a political sense to the dimensions of the warfare against civilians in the former Yugoslavia.
The significant historical factor may well be that the controversy surrounding the debunking of the reports of atrocities in Belgium is one step -- an important one -- in shedding the notion that "civilized" nations set limits to warfare, especially, that warfare is not extended to civilians. (This is a sense that has, of course, not been lost entirely, and in fact exerts considerable force over the conduct of war when waged by the U.S. and European nations, but the theme of the limits of warfare is the subject for another discussion.)
The second factor is the long-lived effects of propaganda on the popular understanding of warfare, including not only the persistence of the messages, but of the reaction against them. Both have the effect of obscuring what happened.