Machine guns are weapons designed to fire an enormous number of bullets as quickly as possible. Instead of requiring their user to pull the trigger to fire each bullet, machine guns are "automatic" weapons. This means they will keep shooting continuously as long as they have bullets and someone holds down the trigger.

In the centuries before the First World War, armies relied on single-shot guns that took a long time to fire and reload. World War I saw the first widespread use of machine guns. As the war progressed, machine guns became more reliable, and their firing power increased. They went from firing 400 rounds a minute to almost 1000 a minute, which averages to 13-15 bullets fired every second.

Machine guns were one of the main reasons trench warfare on the western front lasted for years. Advancing to an enemy trench over No Man's Land became almost suicidal when machine guns were firing dozens of bullets per second. Innovations in rapid fire weren't matched by developments in armor and defense, either. Soldiers in World War I mostly wore cloth uniforms, and at the very beginning of the war they weren't even wearing metal helmets.

The advancements in basic weaponry during the 20th century produced terribly high casualties compared to previous wars, and also changed the way war was waged. Soldiers turned away from using their own bodies as the primary force of combat, and increasingly became operators of these deadly new machines.

Learn more about the history of machine guns in the slideshow below.

Some of the earliest machine guns, like the Gatling gun pictured here, were used during the American Civil War in the 1860s. However, their expense, weight, and clumsiness kept them from becoming standard weapons. World War I was the first European conflict to place machine guns in a primary role. They were used by all sides, and all soldiers feared them. The impersonal, deadly nature of machine gun fire heavily influenced the character of the war. Automatic (and semi-automatic) weapons like machine guns became standard issue for most armies during World War II. They contributed to the enormous death toll of the war. Bundesarchiv, Bild 101I-278-0899-26 / Wehmeyer / CC-BY-SA [CC-BY-SA-3.0-de