Pershing threatened to execute German POW’s on a one to one basis and they backed down.
Of course what is not mentioned is that the US Army learned from direct experience from the Confederate Army just how effective shotguns could be at close range. Before that everyone had only thought of them as sporting arms. Though, they really should have realized how effective they could be given that crude blunderbusses had been used commonly in naval combat to take out densely packed swarms of boarding parties or overcome densely packed defenders when boarding. They put nasty things down the barrel of those - nails, broken glass, etc.
To this day several NATO militaries use shotguns for house to house fighting.
Coming from Pershing, it was not a threat. It was a promise.
A “whiff of grape” had been around a while.