How did machine guns change warfare during World War I?

The introduction of the machine gun fundamentally altered the character of warfare during World War I, earning the conflict the epithet "the machine gun war". By providing sustained, rapid fire—often 500 to 1,000 rounds per minute—these weapons rendered traditional frontal assaults by infantry and cavalry suicidal.1234

The tactical impact of the machine gun included:

• Forced Defensive Stagnation: Because exposed troops could not survive the "storm of steel," armies were forced to seek refuge underground, leading to the development of extensive trench systems.25
• Strengthening the Defense: When combined with barbed wire and rapid-fire artillery, machine guns gave a decisive advantage to the defender. Attacks against these positions often resulted in catastrophic casualties, such as the loss of over 20,000 British soldiers in a single day at the First Battle of the Somme.46
• Tactical Evolution: To counter these defenses, armies eventually moved away from massed frontal charges toward decentralized "fire-and-movement" tactics, utilizing smaller storming parties equipped with lighter, more portable automatic weapons.2