Industrial warfare
With the advent of the industrial age and various technological advancements, lighter, yet powerful and accurate artillery pieces were produced. This gave rise to field artillery which were used on a tactical level to support troops.
Machine guns are fully automatic guns. In this era of warfare they only existed as mounted support weapons, as automatic firearms were not yet developed. Early machine guns as invented by Richard Gatling, were hand cranked but evolved into truly automatic machine guns by Maxim at the end of the era. Machine guns were valued for their ability to smash infantry formations, especially attacking enemy formations when they were dense. This, along with effective field artillery, changed tactics drastically.

Static defense: Static defenses evolved from the use of permanent fortifications that were direct descendants of medieval castles. As artillery improved in destructive power and penetrative ability, more modern fortifications were developed, using first thicker layers of stone, then concrete and steel. After naval artillery developed the turret – a moving cannon platform – land fortifications started to use this method as well. Between the World Wars, France built an "impregnable" underground steel and concrete fortification that ran the length of the German-French border. This Maginot Line  failed to stop German tanks in 1940: they bypassed the fortifications by invading through neighboring Belgium.

Temporary fortifications: As artillery and rifles allowed the killing of enemy personnel at a longer effective range, soldiers started to dig into temporary fortifications. These included massive trenches as used in WWI, and individual soldier-sized "fox holes" which became more common in WWII.

Maneuver warfare: Maneuver had existed throughout military history – from soldiers marching on the field to using horses in cavalry formations. It was not until the advent of mechanized transport over unprepared terrain, such as fields and deserts, using tanks and armored vehicles, that "maneuver warfare" became feasible. First used by the German army in Poland and France in WWII, Blitzkrieg or "lightning war" saw whole armies moved rapidly on tracked and armored fighting vehicles. During the war airborne movement was used, with soldiers dropped to the battlefield by parachute by both the Germans and the Allies. After WWII, developments in helicopters brought a more practical way to transport troops by air.

Armoured warfare
Blitzkrieg
Deep operations

Naval warfare: 

Ironclads and Dreadnoughts: The period after the Napoleonic Wars was one of intensive experimentation with new technology; steam power for ships appeared in the 1810s, improved metallurgy and machining technique produced larger and deadlier guns, and the development of explosive shells, capable of demolishing a wooden ship at a single blow, in turn required the addition of iron armor, which led to ironclads.
The famous battle of the CSS Virginia and USS Monitor in the American Civil War was the duel of ironclads that symbolized the changing times.  Although the battle was inconclusive, nations around the world subsequently raced to convert their fleets to iron, as ironclads had shown themselves to be clearly superior to wooden ships in their ability to withstand enemy fire.
In the late 19th century, naval warfare was revolutionized by Alfred Thayer Mahan's book The Influence of Sea Power upon History.  Mahan argued that in the Anglo-French wars of the 18th and 19th centuries, domination of the sea was the deciding factor in the outcome, and therefore control of seaborne commerce was critical to military victory.  Mahan argued that the best way to achieve naval domination was through large fleets of concentrated capital ships, as opposed to commerce raiders.  His books were closely studied in all the Great Powers, influencing their naval arms race in the years prior to WWI.

As the century came to a close, the familiar modern battleship began to emerge; a steel-armored ship, entirely dependent on steam turbines, and sporting a number of large shell guns mounted in turrets arranged along the centerline of the main deck. The ultimate design was reached in 1906 with HMS Dreadnought, which entirely dispensed with smaller guns, her main guns being sufficient to sink any existing ship of the time.
The Russo-Japanese War and particularly the Battle of Tsushima in 1905 was the first test of the new concepts, resulting in a stunning Japanese victory and the destruction of dozens of Russian ships.  World War I pitted the old Royal Navy against the new navy of Imperial Germany, culminating in the 1916 Battle of Jutland.  Following the war, many nations agreed to limit the size of their fleets in the Washington Naval Treaty and scrapped many of their battleships and cruisers.
Growing tensions of the 1930s restarted the building programs, with even larger ships than before: the Japanese battleship Yamato, launched in 1941, displaced 72,000 tons and mounted 18-inch (46 cm) guns.  This marked the climax of "big gun" warfare, as aircraft would gradually play a larger role in warfare.  By the 1960s, battleships had all-but vanished from the fleets of the world.

Aircraft carriers: Between the world wars, the first aircraft carriers appeared, initially as a way to circumvent the tonnage limits of the Washington Naval Treaty (many of the first carriers were converted battlecruisers).  Though several ships had previously been designed to launch aircraft, the first true "flat-top" carrier was HMS Argus, launched in December 1917.
By the start of WWII, aircraft carriers typically carried three types of aircraft: torpedo bombers, which could also be used for conventional horizontal bombing and reconnaissance; dive bombers, also used for reconnaissance; and fighters for fleet defence and bomber escort duties.  Because of the restricted space on aircraft carriers, these aircraft were almost always small, single-engined warplanes.  The first true demonstration of naval air power was the victory of the Royal Navy at the Battle of Taranto in 1940, which set the stage for Japan's much larger and more famous attack on Pearl Harbor the following year.
Two days after Pearl Harbor, the sinking of HMS Prince of Wales and HMS Repulse, marked the beginning of the end for the battleship era.  Following WWII, aircraft carriers continued to remain key to navies throughout the latter 20th century, moving in the 1950s to jets launched from Supercarriers, behemoths which could displace as much as 100,000 tons.

Submarines: Just as important was the development of submarines to travel underneath the sea, at first for short dives, then later to be able to spend weeks or months underwater powered by a nuclear reactor.  The first successful submarine attack in wartime was in 1864 by the Confederate submarine H.L. Hunley which sank the frigate USS Housatonic.
In both World Wars, submarines primarily exerted their power by sinking merchant ships using torpedoes, in addition to attacks on warships.  All nations practiced unrestricted submarine warfare in which submarines sank merchant ships without warning, but the only successful campaign during this period was America's submarine war against Japan during the Pacific War.  In the 1950s the Cold War inspired the development of ballistic missile submarines, each one loaded with dozens of nuclear-armed missiles and with orders to launch them from sea should the other nation attack.

Aerial warfare: The first use of airplanes in war was the Italo-Turkish War of 1911, when the Italians carried out several reconnaissance and bombing missions.  During WWI both sides made use of balloons and airplanes for reconnaissance and directing artillery fire.  To prevent enemy reconnaissance, some airplane pilots began attacking other airplanes and balloons, first with small arms carried in the cockpit, and later with machine guns mounted on the aircraft.  Both sides also made use of aircraft for bombing, strafing and dropping of propaganda leaflets.
The German air force carried out the first terror bombing raids, using Zeppelins to drop bombs on Britain.  By the end of the war airplanes had become specialised into bombers, fighters, and surveillance aircraft.  Most of these airplanes were biplanes with wooden frames, canvas skins, wire rigging and air-cooled engines.
Between 1918 and 1939, aircraft technology developed very rapidly.  By 1939 military biplanes were in the process of being replaced with metal framed monoplanes, often with stressed skins and liquid cooled engines. Top speeds had tripled; altitudes doubled (and oxygen masks become commonplace); ranges and payloads of bombers increased enormously.
Some theorists, most famously Hugh Trenchard and Giulio Douhet, believed that aircraft would become the dominant military arm in the future, and argued that future wars would be won entirely by the destruction of the enemy's military and industrial capability from the air.  This concept was called strategic bombing. Douhet also argued in The Command of the Air (1921) that future military leaders could avoid falling into bloody World War I-style trench stalemates by using aviation to strike past the enemy's forces directly at their vulnerable civilian population, which Douhet believed would cause these populations to rise up in revolt to stop the bombing.
Others, such as Billy Mitchell, saw the potential of air power to neutralize the striking power of naval surface fleets.  Mitchell himself proved the vulnerability of capital ships to aircraft was finally in 1921 when he commanded a squadron of bombers that sank the ex-German battleship SMS Ostfriesland with aerial bombs. (See Industrial warfare#Naval warfare)
During WWII, there was a debate between strategic bombing and tactical bombing.