The machine gun was not invented for the First World War but it was the first war that it was accepted as a useful piece of military hardware. At the beginning of the war, machine guns were considered by the military as a weapon that did not allow for chivalrous action. It allowed for no heroes, and therefore was not a weapon that should be used. As the war wore on attitudes changed.
The Germans were the first to realize the machine gun was a useful way to make up for their lack of men and made for a great defensive weapon. The machine gun can best be described as "those weapons that could annihilate the enemy as cheaply and as quickly as possible (Ellis, John, History of the Machine Gun, 1975). Especially against the French and British strategy of massing thousands of troops and having them charge straight toward the German trenches.
Amazingly enough, the famous Gatling Gun that made such historic gains in the American Civil War and the Spanish-American war was absent from conflicts and missed the World Wars altogether. In fact, it was not in production at the time and did not return until the 1950's. There seems to be no reason that the Gatling Gun could not have been efficiently converted from a manual to an automatic firing system.
In 1914, the Germans had formed machine gun companies and attached them to their cavalry units. These companies were highly mobile, and were used to plug holes in the German defensive line and counter any Allied gains. The British, still leery of the weapon, used interlocking rifle fire to produce the volume of fire equivalent to a single machine gun. As the war wore on the British could not keep the trenches supplied with enough men to keep the volume of rifle fire to disperse the German infantry charges.
By the end of the war the British had attached one machine gun to every two platoons, allocating another four others for anti-aircraft duties; the French mimicked the British units. The Allies attempted to use gas and artillery to quiet the weapon, but the Germans only countered by giving the machine gunners air tanks and tents for protection against the gas, as well built dug-outs against the artillery fire. The German machine gunners were able to have their weapons functional within 3 minutes after an artillery barrage had passed, and in more that enough time to mow the allied infantry down.
There were many types of machine guns produced for the war. The Allies had adopted the Vickers, Maxims, Hotchkiss and Browning 30 caliber machine guns, both water or air cooled barrels versions. The German military used Lewis and Maxim machine guns.
The Americans would purchase many of the weapons the Allies produced, and sent two successful versions, of their own 30 caliber machine guns to war. 1918 showed the most powerful machine gun produced for the war, and would not change for the next 85 years, the 50 caliber air cooled machine gun. These Browning built weapon was held in reserve until General John J. "Blackjack" Pershing felt the U.S. units had enough to produce the shock effect on the battlefield. The only problem with the weapon was that it could only be fired continuously for 100 rounds before having to allow the barrel to cool, but the cooling only took a few second.
The machine gun was a weapon highly disliked by all combatants of the war, but used because of its deadly efficiency. "One machine gun, properly placed, can hold up a battalion at times." (Levine, Isaac Don, The Pioneer of Air Power Levine 1943).
World War I produced many weapons that would affect the future of combat. If it was not for the armistice, the First World War would have seen its technology kill more soldiers and civilians in 1918, than possible in the whole war. It could be said that the Great War sped up research and development into technology by twenty-five years.