From Wiki under Winchester 1897 article.
(sources at the article)
Although the Model 1897 was popular with American troops in World War I, it wasn’t so popular with the German troops. “On 19 September 1918, the German government issued a diplomatic protest against the American use of shotguns, alleging that the shotgun was prohibited by the law of war.” A part of the German protest read; “It is especially forbidden to employ arms, projections, or materials calculated to cause unnecessary suffering”.”This is the only known occasion in which the legality of actual combat use of the shotgun has been raised. However, the United States interpreted their use of the shotgun differently than Germany. The Judge Advocate General of the Army, Secretary of State Robert Lansing carefully considered and reviewed the applicable law and promptly rejected the German protest.
The rejection of their protest greatly upset the German forces, because they believed they were treated unjustly in the war. Shortly after the protest was rejected, Germany issued threats that they would punish all captured American soldiers that were found to be armed with a shotgun. This led to the United States issuing a retaliation threat, stating that any measures unjustly taken against captured American soldiers would lead to an equal act by the United States on captured German soldiers.
I carried a 12 gauge Remington pump with shortened barrel in RVN. Someone told me it was a violation of the Geneva Conventions. I ignored him. Never did find out if he was right, but that baby was attached to me for 13 months....