Staff Writer (Updated: 4/8/2016):
When America formally entered World War 1 on April 6th, 1917, it had little in the way of a local wartime arms industry and therefore had to make do with Triple Entente material - this included tanks, aircraft, rifles, machine guns and the all-important hand grenade from allies such as France and the United Kingdom. Hand grenades proved very useful in the dislodging of enemies dug into the fixed battle lines that were made up of miles of trenches found across the European landscape. However, great care needed to be taken to ensure that the grenade operator did not unnecessarily expose himself to enemy fire when throwing or when bracing for detonation - or having his live grenade returned back to him by the enemy with expected disastrous results. American forces entering the war were initially issued the "F1" series hand grenade from the French or the "Mills Bomb" hand grenade from stocks originating in the United Kingdom. Over time, the need for an indigenous hand grenade became apparent (as did the national pride of sending one's own troops into combat with indigenous American weapons), leading United States authorities to commission research into such a design.
Engineers of the US Army Trench Warfare Section responded with the "Mk 1" in August of 1917, a hand grenade not unlike the French F1 series. A contract for 5,000 units soon followed and orders eventually ballooned to 68,000,000 within a few short weeks thanks to an enthusiastic and well-organized American war effort at home. Orders subsequently were fulfilled and these grenades were shipped overseas to awaiting American units. However, after just a short time in the field, American authorities (based on in-the-field experience reports) delivered a scathing evaluation on the function of their Mk 1s in April of 1918. Apparently, the grenade design was "too" fool-proof to the point that live grenades could be returned to the thrower due to the complex five-step arming process. In the heat of battle, it was simply too easy for an American soldier to complete only some of the arming process before throwing the grenade without much time to think - resulting in the battle-hardened enemy fully arming the grenade and returning it back to the American position. The full process required the operator to pull a safety pin, remove a cap off of the grenade top, ready a lever on the grenade to begin the countdown on the fuse, throw the grenade a safe distance away and find cover from the ensuing blast of fragments. A timed friction fuse took over to complete the detonation process. As can be expected, production of the new grenade was completely stopped for the issue to be fully addressed and the timing proving somewhat critical for the American Army was soon to enter heavy direct enemy action by this time in the war.
By August 1st of 1918, the redesign had completed and a simpler arming process was installed. Production ensued and by the Armistice on November 11th, 1918, some 17,447,245 units were made available, some even using unused parts from existing Mk 1 production until assembly lines were retooled for the new design. However, this improved form - the "Mk 2" - arrived simply too late to take part in any full-scale combat actions from there on. Regardless, the world war was over and the Americans had taken away a good deal of experience in the design and production of their own hand grenade - concepts that carried over into designs that covered well over a century.