Gleaned from the internet
" In September 1918, the German government issued a diplomatic protest, complaining that the Model 97 Trench Gun was illegal because “it is especially forbidden to employ arms, projections, or materials calculated to cause unnecessary suffering” as defined in the 1907 Hague Convention respecting the Laws and Customs of War on Land.
When the Americans rejected this, the German high command then threatened to execute any soldier caught with a Trench Gun or even just Trench Gun shells.
General Pershing replied that, henceforth, any Germans caught with flamethrowers or saw-bladed bayonets would be lined up and shot. As far as is known, no American or German POWs were executed under such circumstances "