The Garand was the brainchild of John Garand, who developed it while trying working on a project to build a gas-powered semi-automatic rifle in Springfield Armory. The Garand became was finished in a decade, and replaced the bolt-action rifles that the Army was using prior. By 1936, contracts had been secured for the major Branches, and the weapon was in production.
The Garand was evolutionary, but not quite revolutionary. It wasn’t a technology invented that made headlines, and it certainly didn’t have as drastic effect as the airplane, the tank, or the nuclear bomb. However, the Garand took existing improvements to rifles of the era, and improved them. Firstly, it was gas-powered, allowing for recoil to be more controlled, and the gas released to power the mechanism that loaded another round into the breech. This same mechanic also made the rifle semi-automatic, meaning that a soldier didn’t have to hand-load ammunition (as they did in the previous century), and didn’t have to operate a bolt on the receiver (as they did during the first world war).
Overall, these improvements made the American Rifleman quicker, and generally more accurate. With the inclusion of magazine fed ammunition (as opposed to the 5-round clip originally designed for the weapon), American Infantry could bring even more volume of fire to the enemy before having to reload the weapon. This is in direct opposition to the Type 99 Rifle used by the Japanese, that had a 5 round clip, but was operated manually by bolt action. The Germans had the Gewehr 1941, however, it proved unreliable, and most German Infantry were armed with bolt-action Karabineers. The Soviet Union had the Tokarev, which had been a shock to the Germans (as they seemed to be more common issue for Soviet Infantry), and the British had the Lee Enfield which was bolt-action. Therefore, of all the Allies, only the the Soviets had a comparable infantry rifle. Of the Axis, only the Germans had a semi-automatic rifle, but it suffered from bad design, and over 6000 produced, 1200 were returned as ‘un-serviceable’.
Overall, this drastic difference in infantry capability changed how the war was fought. The standard American Infantry could lay down much more suppressive and killing fire, in higher volumes, and with less exertion than his German counter-part. Due to the fact that the breach remained closed, and rounds chambered themselves, he never had to worry about in-climate weather, or turning a corner without having a round in the breech. This reduced the combat load, and made them far more versatile combatants.