Trench warfare
Trench warfare prevailed on the Western Front from late 1914 until the Germans launched their Spring Offensive on 21 March 1918. After the buildup of forces in 1915, the Western Front became a stalemated struggle between equals, to be decided by attrition. Frontal assaults, and their associated casualties, became inevitable because the continuous trench lines had no open flanks. Casualties of the defenders matched those of the attackers, as vast reserves were expended in costly counter-attacks or exposed to the attacker's massed artillery. There were periods in which rigid trench warfare broke down, such as during the Battle of the Somme, but the lines never moved very far. The war would be won by the side that was able to commit the last reserves to the Western Front.
Combat shotgun
The development of the repeating pump-action shotguns in the 1890s led to their use by US Marines in the Philippines insurrections and by General "Black Jack" Pershing's pursuit of Pancho Villa, and "riot" shotguns quickly gained favor with civilian police units, but the modern concept of the combat shotgun was fully developed by the American Expeditionary Forces during World War I. The "trench gun", as it was called, was a short-barreled pump action shotgun loaded with 6 rounds containing antimony hardened 00 buckshot, and equipped with a bayonet. The M1897 and M1912 also could be slam fired: the weapon having no trigger disconnector, shells could be fired one after the other simply by working the slide if the trigger was held down, though in the heat of combat one could easily short-stroke the weapon and jam it. When fighting within a trench, the shorter shotgun could be rapidly turned and fired in both directions along the trench axis. The shotguns elicited a diplomatic protest from the German government, claiming the shotguns caused excessive injury, and that any troops found in possession of them would be subject to execution. The US Government rejected the claims, and threatened reprisals in kind if any US troops were executed for possession of a shotgun.