Explanation: After some three years of European powers brutally bludgeoning, bayoneting, machine-gunning, and using chemical weapons on each other in WW1, the Americans entered the war. We, of course, immediately indulged in the same bludgeoning, bayoneting, machine-gunning, and chemical weapons as every else did, but we also brought a large quantity of pump-action shotguns to the fight, with brutal effect in the tight confines of the trenches.
The German Empire lodged a formal complaint that the use of shotguns was against the laws of war due to the inhumane nature of the weapon. "Lol", said everyone else, "lmao"
"Jerry" is a nickname for German soldiers; just as British troops were sometimes called "Tommy".