Gunpowder, which came into use in the late 13th or early 14th century, completely changed warfare and had a dramatic impact on the organization and tactics of armies, and also played a major role in ending feudalism. Gunpowder-based artillery made the rulers more powerful. A knight's armor was little protection against firearms, and artillery eventually doomed the castle as a fortification. Artillery was too costly to be privately owned, and it became the property of the king. He employed professional cannoneers to man the guns; this was the beginning of standing armies.

At the end of the 15th century Spain hired professional soldiers and formed them into tercios, units composed of musketeers (soldiers armed with muskets) and pikemen (armed with pikes). These units were the first infantry, in the modern sense. By this time the long dominance of mounted knights, which had begun to crumble as early as the Hundred Years' War (1337–1453), was ended.

In the early 17th century King Gustavus Adolphus of Sweden organized what is con-sidered the first modern army. He formed companies of 72 musketeers and 54 pikemen to fight in lines six files deep instead of in solid masses. Companies were assigned to larger units called battalions, regiments, and brigades. Cuirassiers, cavalry wearing cuirasses (light armor in the form of a vest), were used as shock troops. Dragoons were mounted infantry. He adopted lighter guns and carriages to make his artillery more mobile and used it closely associated with his infantry.

France during the reign of Louis XIV (1643–1715) produced notable military organizers and theorists. The Marquis de Louvois as minister of war was the first to set up an organization to handle army supply. He also instituted inspections to insure uniformity of discipline. One of his inspectors, Jean Martinet, left his name in military language (and in common speech as well) for a person who insists on petty details of discipline. The Marquis de Vauban was the first great military engineer. His methods of constructing fortresses—and of besieging them—were followed for several generations.

Frederick the Great of Prussia took over many French developments, notably the bayonet, which had been developed in the early 17th century. He particularly emphasized a warfare of rapid movement. To his army he added hussars (light cavalry for scouting and skirmishing) and horse artillery (in which all men who served the guns were mounted). Like other armies of this period, his troops fought in straight lines and maneuvered on command as if on the parade ground. He developed the "oblique order," a superior method of maneuvering and controlling these rigid lines.

The British brought these drill-book tactics with them to America. The French and Indians, however, fought from behind cover and the rigid, unbroken lines of British soldiers proved easy targets for them in their defeat of forces under General Edward Braddock in 1755. Lord Augustus Howe, observing the success of the Indian-type tactics of Robert Rogers' group of raiders called the Rangers, suggested that the British adopt formations of light infantry. This concept led to the use of a single line of skirmishers—infantrymen sent out ahead of the main body of troops to seek out the enemy.

In France artillery was standardized by Jean Baptiste de Gribeauval, who developed lighter and more maneuverable guns. The Duke of Broglie and Count Jacques Antoine Hippolyte de Guibert suggested forming various kinds of troops into divisions and grouping the divisions in larger units called corps. Pierre-Joseph Bourcet proposed a staff corps to make maps and prepare war plans. In Prussia Frederick the Great started training of staff officers.

A military draft system was adopted in 1793 by the French government. Napoleon successfully applied the ideas of the military theorists who had gone before him. He became a master of rapid movement, of surprise, and of maneuver to bring force to bear on enemy weakness. France during the Industrial Revolution had adopted conscription, creating mass armies and the concept of "the nation in arms" in which all citizens are involved in a war. The huge armies of the 19th century greatly complicated the problems of logistics—supply and movement. Napoleon coined the phrase "An army moves on its stomach."

After defeating Prussia at Jena in 1806 Napoleon imposed a limit of 42,000 men on the Prussian army. Gerhard Johann David von Scharnhorst evaded this limitation by training men for military service and then passing them into the reserve; thus the army itself was small but it was backed by trained men who could be called into service when needed. This was the first effective reserve system, and it proved its value in the eventual defeat of Napoleon.

By the time of the American Civil War firepower had so increased because of improved firearms that the stand-up fight of lines of infantry was permanently abandoned. By 1864 soldiers, even without orders, dug trenches to protect themselves, and the last year of the war was fought largely in field fortifications. In this war railroads were first used to move and supply troops, and the telegraph was first used to transmit orders.

The value of efficient staff planning was demonstrated in the Franco-Prussian War, 1870–71, in which the German armies quickly defeated the French. In this war were made the last charges by large masses of cavalry. Development of rapid-fire, breech-loading arms spelled the doom of the horse on the battlefield.

Machine guns and barbed-wire entanglements were innovations in the Spanish-American War, 1898. In the Boer War, 1899–1902, British forces found that modern arms could not easily overcome guerrilla forces.

In World War I, 1914–18, machine guns and greatly increased use of rapid-fire artillery drove the armies underground, and most of the fighting for four years on the Western Front was from elaborate trench systems. Railways were used to carry millions of soldiers to the fighting fronts and by motor trucks to haul supplies from railroads to the front lines.

The Germans introduced poison gas in an effort to break through, but gas proved a peril to the user as well as to the opponent. It was rarely tried after World War I. In 1916 Britain introduced the tank. It was slow-moving and lightly armed, but it proved moderately effective. In World War I also came the first use of the airplane in warfare—at first only for reconnaissance, but later for bombing behind the lines and for support of troop movements.

In World War II, from 1939-1945, greatly improved tanks and airplanes caused a revolution in military tactics. The Allies, who included Canada, the Soviet Union, the United Kingdom, and the United States, fought the Axis powers, which included Germany and Japan in the war. There was no trench warfare. The Germans developed the blitzkrieg (lightning war), in which tanks, fast-moving columns of troops, and supporting aircraft fought as a unit and quickly overran opposing forces. (These tactics were later adopted by the Allies, notably under the American general George S. Patton.) The airplane was put to new uses in World War II—to carry troops into battle and for long-range strategic bombing.

In World War II armies in large-scale airborne operations were first used. The first successful paratroop invasion in May 1941 was launched by the Germans, when they seized the island of Crete. When Allied armies landed in Normandy, in northern France, the largest combined land, sea, and air attack in history occurred on June 6, 1944.

In the Pacific, the war was amphibious. First the Japanese seized Allied territory by seaborne invasions and then the Allies took key islands with a series of brief but costly amphibious assaults. These campaigns demonstrated the importance of close cooperation between land, sea, and air forces.

The atomic bomb was used in the last stages of the war against Japan in Hiroshima and Nagasaki, but was not used in the numerous "limited" wars that followed. Although these wars were fought largely with weapons like those used in World War II, there were such new developments as guided missiles, helicopters, and jet aircraft. These weapons changed the thinking of many experts about the tactical use of nuclear weapons. Such cannons and missiles could launch nuclear warheads at a large body of troops, a supply dump, or any other target within the weapons' range. Except in the Korean War (1950–53) the "limited" wars rarely involved full-scale combat between large masses of troops; the trend, rather, was toward guerrilla warfare, in which small units of fast-moving troops strike quickly and then withdraw.

During its involvement in the Vietnam War, from 1965 to 1973, the United States Army relied heavily on conventional warfare tactics. To fight Communist guerrillas in the jungles of Vietnam, these tactics were considered the most effective way by the American army. The Vietnamese guerrillas relied on surprise and mobility and avoided major battles in the open, where heavy U.S. firepower could be decisive. In 1956, 1967, and 1973, the Arab and Israeli forces fought wars using conventional weapons. Both the sides resembled the tactics of World War II, in which aircraft and tanks spearheaded attacks and were followed into battle by infantry.

In the late 20th century, the armies of developed nations have applied tactics like fighting brief fights, intense wars that usually decide a conflict quickly. Development in computer technology and other electronics also made a massive impact on armies. Today, instant satellite photographs, real-time images of battlefields, and worldwide communications with political and military leaders are easily accessible by many armies. On very short notice, now Armies can concentrate on strategic mobility, the ability to quickly move troops, supplies, and equipment anywhere. This can be done using fast troop transport ships and large, troop-carrying jet airplanes. In only two months, the United States deployed nearly 200,000 troops and their equipment to Saudi Arabia during the Operation Desert Storm in 1990.