The First World War (1914 to 18) has a close association with static trench warfare dominated by heavy artillery and machine guns, but the conflict witnessed many entirely new developments in weaponry as all sides desperately sought to outdo the enemy and then, very often, invent countermeasures to these new and terrible devices of destruction. Innovative weapons of WWI include the flamethrower, hand grenade, gas shell, tank, long-range bomber plane, mine, torpedo, and depth charge.

Flamethrowers were first developed by the German Army, although they were then adopted by others. Some flamethrowers required three men to carry and operate, but the development of a portable version allowed a single soldier to carry a weapon that could cause devastation if he managed to get in or near the enemy's trench system. The weapon was used by specially trained assault units known as 'storm troops'. The device used gas to push out fuel under pressure, which was ignited when it reached the nozzle. The flaming petrol could shoot out to a distance of up to 36 metres (40 yards). This impressive-looking weapon was not quite as effective as hoped for. Apart from the high risk of the bearer blowing himself up, the most serious flaw was that the soldiers armed with flamethrowers became the first (and most visible) targets for enemy fire, and so using one became almost a form of suicide.

The first grenades to be used in WWI were primitive homemade affairs where soldiers used their imagination and what materials they had at hand, such as old jam or tobacco tins, to make an explosive device that could be thrown at the enemy. Tens of millions of hand grenades were manufactured through WWI as mass production of the devices began in 1915.

The German hand grenade used a wooden handle and so became known as a stick grenade (Steilhandgranate). The explosives in the tin can top were primed by pulling a wire in the handle and would go off after either 5.5 or 7 seconds. The stick design was so successful, it was still widely used through WWII (1939 to 45). The French Army, in contrast, developed the 'bracelet' grenade, which had a leather strap attached to the primer that could be worn around the wrist. The British, meanwhile, went for a smaller device that looked like a miniature pineapple with its deeply grooved sides for extra grip. First called a Mills Bomb, pulling out a pin and releasing a lever primed the explosives, which went off after about five seconds. As tanks became more common, more powerful anti-tank grenades were developed. There were also grenades that, instead of being filled with metal fragments or purely explosive material, contained smoke to hide troop movements, illuminating material for use at night, or gas to poison or confuse the enemy. Not only thrown by hand, British and German grenades could also be fired from a rifle using a wooden adapter.

The terrors of poison gas in WWI first appeared in April 1915 at the Second Battle of Ypres. Deployed by the German Army, the gas was simply released from canisters on the ground with the hope that the wind would carry it to the enemy. Other armies soon adopted poisonous gas, although the fact that Germany had started its use was not lost on the issuers of Allied propaganda. Then, from July 1915, German engineers developed the gas shell, which allowed artillery to fire the lethal substance over much greater distances and right into the midst of the enemy, whatever the local weather conditions. Gas shells contained a glass bottle with poisonous liquid inside. When the shell landed, the glass smashed, and so the liquid was released and made contact with the air, thereby changing into gas. Mortars were also used to fire gas shells, often set up in long rows to lay a concentrated carpet of gas in a specific area occupied by the enemy, a necessary action if the wind did not disperse the gas harmlessly away.

It did not seem to bother anyone that this new weapon was contrary to the 1899 Hague Convention, which prohibited projectiles that contained gas. Gases used included non-lethal tear gas (lachrymatory); chlorine gas, which burnt the lining of the lungs and choked the victim to death; mustard gas (dichlorethylsulphide), which impeded vision, caused large painful blisters, and destroyed the lining of the victim's air tracts; and colourless phosgene gas (aka Green Cross gas), which was the deadliest of all. Chemical weapons did not prove strategically decisive since countermeasures like gas masks frequently negated their effects. Nevertheless, poisonous gas still accounted for one million casualties in WWI.

In the war at sea, submarines became a lethal menace to enemy naval vessels and merchant shipping thanks to the use of torpedoes. The torpedo was a self-propelled missile that was difficult for those on the target vessel to detect or avoid. Travelling just under the surface, the torpedo exploded on contact. Some torpedoes included a mechanism that could control the depth and direction the weapon travelled. Measuring from 5.3 to 6.7 m (17.5 to 22 ft) in length, the fastest could travel at 44 knots. The maximum effective range was around 9 km (10,000 yds). The typical German submarine or U-boat carried six torpedoes, which could be fired either from the bow or stern of the vessel. Torpedoes were used by all sides in the war, and they could also be launched from ordinary surface ships and aircraft. The torpedo turned out to be the most successful weapon at sea during WWI, accounting for far more victims than naval guns or mines.

Torpedoes were so effective that various defence mechanisms were devised, such as using armed convoys for merchant shipping and depth charges (see below). One of the more curious defences for ships developed during the war was dazzle camouflage, designed to confuse the outline of a ship by having an unusual combination of geometrical shapes painted on the sides of ships. It became difficult for a U-boat commander to correctly identify a ship or even where its bow and stern ended. The initial designs were made by the British artist John Everett, while the actual application was supervised by the vorticist artist Edward Wadsworth. The United States and then other countries also adopted this type of camouflage. Destroyers, which were tasked with hunting down submarines, were the most typical vessel type to receive dazzle camouflage.

The threat of torpedo-carrying submarines led to, as noted above, further innovations to counter them. The depth charge was first used in 1916. Shaped like a barrel, the device was rolled off the back or sides of a ship, and the explosives inside were detonated when it sank to a certain depth. The depth setting could be changed depending on the actions of the enemy submarine and was activated by a hydrostatic valve, which measured water pressure (which varies depending on depth). Another innovation that assisted the war on submarines was the hydrophone, which, although not yet enjoying a great range of operation, could detect sounds in the deep and so allow the hunted surface vessel to become the hunter and drop depth charges more precisely.

Another method to combat enemy submarines and shipping in general was to lay minefields in areas of the sea most likely to be entered. All sides used this method of defence, and a variety of types was developed. The most common mine was the contact mine, which was usually attached to the seabed and could be detonated when a vessel hit it, depressing one of many protruding detonators. Another type was the controlled mine, which could be detonated remotely by someone on the shore. This type was particularly useful to defend harbours against attack. British scientists developed a third type, the magnetic mine, which was detonated by the magnetic field of a ship moving above it. The German Navy, meanwhile, became expert at laying mines and developed submarines that could do this.

Mines accounted for hundreds of sinkings and became such a hazard that a new type of ship was developed to find and neutralise them, the minesweeper. This type of ship, which usually worked in pairs, dragged a wire that cut the tethering cable of any mines it went over, thus making them bob up to the surface, where they could be destroyed. Minesweepers were, in turn, responded to by engineers, who developed, by the end of the war, mines that could be sunk to a set depth from the surface and so need not be tethered.

The first tanks to appear in WWI were British at the First Battle of the Somme (July-Nov 1916). Initially imagined as a mere support for cavalry, as these metal monsters with caterpillar tracks evolved, it was finally realised that they could be a valuable weapon in their own right, especially when used together in large numbers. Early tanks were slow-moving tin cans that either broke down or guaranteed a horrible death for their crews, but by the final year of the war, tanks had greatly improved, and they began to overwhelm enemy trench systems and even win battles. The first tanks-against-tanks battle came in April 1918, a portent of future warfare on land.

With armour plating, one or two heavy guns, and several machine guns, the tank suddenly made warfare a whole lot more mobile. The British Mark IV tank, which had a distinctive lozenge shape, was the most-built British tank of the war. It had a crew of eight men and weighed up to 30 tons. This tank could only reach a top speed of 6.5 km/h (4 mph). The lighter Medium Mark 1 tank was capable of 13 km/h (8 mph) and had a range of 128 km (80 mi).

The French Saint Chamond tank had a crew of nine, four machine guns, and a 75-mm (3-inch) gun, but it proved unreliable in just about every department. Much better was the Renault FT-17 light tank. Louis Renault personally supervised design and production, and this tank was the first to have a fully rotating turret, a design feature most tank designers have copied ever since. The FT-17 was such a success that it was adopted by US forces during WWI, and it was still being used by several other armies at the start of WWII.

Despite countermeasures such as armour-piercing bullets and the invention of the anti-tank gun, tanks gradually began to make their mark as the new weapon of warfare. The years-long, essentially static trench warfare on WWI's Western Front finally became more dynamic. Oddly, the German Army was a slow convertor to the capabilities of tanks as Britain and France led the way, but this situation would be entirely reversed by the beginning of the Second World War.

While WWI saw the rapid development of airplane design and the possibility of fast and high-flying fighter planes, the arrival of long-range bombers perhaps had the greatest significance on how war was conducted, particularly from a civilian point of view. Now, towns and cities a long way from the fighting front became vulnerable to direct attack. The Zeppelin bombing raids of WWI had caused a sensation, but these giant airships were too slow, too few, and too vulnerable to attacks from airplanes to make any significant contribution to the war. Squadrons of bomber planes targeted enemy infrastructure like utility services, transport networks, supply dumps, and weapons factories.

The Italian Air Force used Caproni bombers, large, reliable aircraft with a range of several hours and remarkably tough to bring down. Caproni bombers could be, like some aircraft of other nations, converted for use at sea by fitting floats instead of wheels. Seaplanes were frequently used to drop torpedoes.

British Handley Page bombers had twin Rolls-Royce engines, a range of over 1,125 km (700 mi), and could carry a bomb up to 1,524 kg (3,360 lbs) in weight, although this was exceptional. French bombers started small by dropping metal darts, but they soon moved up to bigger bombs and even managed to drop converted artillery shells. The biggest German bomb was the aerodynamically designed P.u.W, which went up to 1,000 kg (2,200 lbs). Early German bombing raids in Britain were carried out by Gotha airplanes, and the name Gotha was applied by the British thereafter to all German bombers, whatever their manufacturer. The Gotha IV, powered by Benz engines, had a massive wingspan of 23.7 m (78 ft) and had a range of over 480 km (300 mi).

Continuing the trend of still-familiar manufacturer names, the French Breguet bomber was built by Michelin and had a Renault engine. The Breguet 14 was the best French bomber that could reach 185 km/h (115 mph) and could carry a rack of 32 8-kg (17.5-lb) bombs.

Russian Sikorsky bombers were the first to have four engines, which meant they could carry heavier bomb loads. Typically, Sikorsky bombers had a crew of nine, three or four machine guns as defence, and a range of 640 km (400 mi). Germany only managed to shoot down one Sikorsky bomber throughout the entire war, and their design was so successful that they were used throughout the Russian Civil War.

Ultimately, improvements in air defence, particularly anti-aircraft guns and much faster fighter planes, forced most bombers to conduct raids at night, which greatly reduced their accuracy. In the end, airplane bombers, like the Zeppelins, only had a marginal effect in the war, too, but their rapid design evolution meant they would, along with the tank, become very much the weapon of the future.