Technology changed significantly between the Civil War and World War I. In the Civil War, metal-plated shapes were an anomaly and used in river warfare only; by World War I, these ships became the norm and the massive battleships were considered the pride of the major powers' navies. All the major powers of World War I used submarines to sink merchant shipping, and this altered the rules of war, as a submarine had to hide under the sea before attack. During the Civil War, it was standard procedure to give crews time to leave the ship before it was attacked. German unrestricted submarine warfare was one of the reasons that the United States joined World War I.

The airplane brought a new dimension to warfare that was not present during the Civil War. While the Union used observation balloons during the Peninsula Campaign, these were not the norm, and both armies used cavalry to scout enemy lines. The airplane, first developed in 1903, became a useful war weapon for reconnaissance and even early bombing, though the first planes could not carry massive bomb loads. Civilians became targets with the German zeppelin raids as well. By 1917, planes were becoming more adept at attacking ground targets and enemy planes—all of this would be a precursor for the next world war, which would prove even more destructive. The airplane and the submarine meant that the average worker now joined the front-line infantryman as a target in warfare; this change in strategy made major cities important targets just as much as objectives on the field of battle.

By the end of the Civil War, both armies were mainly using rifles. The breechloader gave the infantryman more firepower than ever before. By the time of World War I, the machine gun crew gave the defender a distinct advantage, and both sides began to dig elaborate trench systems protected by yards of barbed wire, another American invention. This led to a stalemate on the western front, which lasted from the end of 1914 until the fall of 1918. Most infantry carried bolt-action rifles, which were more accurate and afforded a greater rate of fire than the rifles of the Civil War, though these rifles did not use the .58 caliber bullet like many of the Civil War weapons. The new bullet offered more power and both sides experimented with hollow point rounds that left large exit wounds. Both the Central and Allied powers were desperate to break through the trenches and resorted to poison gas, a byproduct of industrial processes. Chlorine and mustard gas became a terror in the trenches, and both sides developed gas masks. The British developed the tank, but tactics did not catch up with technology. In many cases, the tank did not have infantry support behind it in order to follow up on the gains it made. Both sides developed massive artillery to blast through trenches and barbed wire systems—new developments in explosives meant that the artillery had a greater range than ever before.

The technology of World War I outpaced the tactics. Soldiers were lost by the thousands in frontal attacks on trench systems. Sanitation still had not changed much from the Civil War; soldiers died of influenza and infection because conditions in the trenches were horrible. The trenches were a breeding ground for both disease and vermin, and ultimately this killed more soldiers than the bullets.