The First World War (1914 to 18) witnessed an arsenal of new weapons as all sides were desperate to gain an advantage, particularly in the static trench warfare of the Western Front. There were some old tried-and-tested weapons like the Lee-Enfield rifle and massively armed battleships, but innovations included hand grenades, heavy artillery, armoured cars, tanks, and fighter planes. Below are some of the successes employed by the British armed forces as designers searched for ever more destructive weapons that could turn the tide of the war in their favour.

The standard-issue rifle to British Army infantry was the Lee-Enfield rifle. This bolt-action rifle of joint US-British design had been in production in one form or another since 1888. It was a reliable and fast-loading gun with a 10-round or a 5-round magazine of .303 ammunition. An experienced soldier could fire off from 15 to 20 rounds a minute. The rifle was effective up to a range of around 600 yards (550 metres) and could be fitted with an M1907 bayonet, a knife with a 17-inch (43-cm) long blade and handle, for brutal close-quarter fighting. The Lee-Enfield was such a success that it was still being used by the army in the 1950s.

The machine gun probably claimed more lives than any other weapon in the trench warfare of the Western Front (although artillery caused more wounds). Specialised units operated machine guns, and these belonged to the Machine Gun Corps, created in 1915. A US-designed light machine gun, the Lewis gun, proved remarkably adaptable and was used by the British Army and Royal Flying Corps. Manufactured in Britain and Belgium, the gun used gas to fire 450 to 500 rounds a minute. This impressive rate of fire made a single machine gun the equivalent of 100 rifles.

The distinctive flat circular magazine of the Lewis gun held either 47 or 97 rounds of .303 ammunition. With a steel air-cooling jacket around the barrel, the gun was heavy and required a bipod for support. The Lewis gun was easy to manufacture on a large scale and was also versatile, being mounted in armoured cars, tanks, motorcycle and side-car units, airships, and aircraft. In all, 50,000 were produced in World War I. Like the Lee-Enfield rifle, the Lewis machine gun was used right through the Second World War (1939 to 45).

The Lewis gun did have a serious rival in the Vickers machine gun. More complex to manufacture, the Vickers was lighter and, unlike the Lewis gun, could be synchronised to fire through the propeller arc of aircraft. By 1918, 5,000 Vickers machine guns were being built every month, and they saw just as wide and varied a use as the Lewis gun. A Vickers machine gun fired .303 bullets, which were fed via a canvas belt holding 250 cartridges. The Vickers gun became the British Army's favourite machine gun and was in service for half a century.

The first British hand grenade was the Mills Bomb, in use from the summer of 1915. Made of a cast iron body containing metal fragments and explosives, a pin had to be pulled and a lever released for the device to explode after four or five seconds. To ensure the grenade was not accidentally dropped, the outer casing was given a distinct grooved pattern. Grenades could also be fired from rifles using a wooden stick and a blank round. A very useful weapon to stun the enemy when attacking their trenches, as many as 70 million grenades were thrown by all sides during the war.

As the Western Front became bogged down in a static trench war, artillery became even more important as a way to harm a well-protected enemy and destroy dense fields of barbed wire. The bigger the guns the better. The British relied on the 12-inch (30.5-cm) siege howitzer, which could fire a shell weighing 750 lbs (340 kg) over a distance of 14,300 yards (13,075 m). More common was the 3-inch (75-mm) field gun, which was also used as a mobile anti-aircraft gun when mounted on a road vehicle. The standard British artillery piece was the 18-pounder (3.3-inch / 84-mm calibre) field gun, which had a maximum range of 6,525 yards (5,966 m).

Accuracy was something of an elusive dream, but gradually, as the war progressed, improvements were made, particularly in combining reconnaissance and command control. A technique was developed called a 'creeping barrage' where artillery was fired to keep pace with advancing infantry, providing a protective 'curtain of fire'. Another important development was to use high explosives, which made the shells much more destructive. The British Army used Amatol, a mixture of trinitrotoluene (TNT) and ammonium nitrate.

The challenge for effective artillery was solving the logistics of getting sufficient ammunition to the front. A single large battle could use up millions of shells, which had to be replaced using trains and horses. Ammunition factories, often employing women working 12-hour shifts, operated around the clock to keep up with the demand. "By the end of the war it is estimated that the British Army had fired an astonishing 84 million rounds from their artillery pieces (that is an average of over 2,000 rounds every hour)" (Yorke, 58).

Experiments had been made with armour-plated cars before the war, but the first purpose-designed vehicles were only produced from 1914. An armoured rotating turret contained a single machine gun or a 1.8-inch (47-mm) gun and was placed within a reinforced chassis, which itself had armour plating. These vehicles could withstand small arms fire and were principally used for reconnaissance. The most successful models were made by Rolls-Royce, Lanchester, and Austin. Design improvements included doubling the back wheels, strengthening axles, adding an extra gun, four-wheel drive and half-tracks to cope with a wider variety of terrains. The Rolls-Royce version was the best and had a top speed of 50 mph (80 km/h) and a range of 150 miles (250 km). The armoured car was most useful on more mobile fronts like the Eastern Front, the Middle East, and in East Africa. Many WWI armoured cars were robust enough to still see service in WWII.

The first tanks to appear in WWI were British at the First Battle of the Somme (July-Nov 1916). These metal giants were first envisaged as a support for cavalry. Tanks were officially called 'landships' but acquired their more familiar name because they looked like water cisterns on wheels, and the authorities were keen to keep their real purpose a secret from the enemy. By the time it was realised that tanks were best used in groups and as a weapon in their own right, the war was almost over.

British tanks had a distinct rhomboid shape when seen from the side, and their tracks were exposed all around. Armour on later varieties was 12 mm thick. There were two types, the 'males' with one 6-pounder gun on each side, and the 'females' with two machine guns on each side and one at the front. Unlike later WWII tanks with a gun in the top turret, this design was intended to give them a lower centre of gravity and make them less vulnerable to enemy fire.

WWI tanks were slow, difficult to steer, and often incapable of crossing gaps in the terrain, such as enemy trenches. The British Mark IV was over 26 feet (8 m) long, had a crew of eight men inside, and weighed up to 30 tons. This tank could only reach a top speed of 4 mph (6.5 km/h). The lighter Medium Mark 1 tank was capable of 8 mph and had a range of 80 miles (128 km). Conditions inside these primitive tanks were horrendous. Almost suffocated by the fumes from the twin engines and the guns, the crew had to communicate using hand signals since it was so noisy inside.

Tanks were often mechanically unreliable, but they were used effectively in several battles in the final year of the war, notably the Battle of Amiens, where 600 tanks helped the Allies capture 20 miles (32 km) of enemy territory. In contrast to the Allies, the German Army was a late adopter of the tank and had produced only 20 at the war's end (compared to Britain's war production of 2,617). This situation would be reversed in the Second World War.

The most powerful naval vessel was the dreadnought-class battleship, which had an armament of eight or ten 12-inch (30.5-cm) guns and a top speed of 21 knots. Britain had 20 dreadnoughts at the war's beginning (Germany had 15). Larger variations, like the Iron Duke class (named after the Duke of Wellington), had the first anti-aircraft guns to be fitted to ships. The Queen Elizabeth class had the biggest guns (15 in or 38 cm).

Used with extreme caution for fear of losing such a costly weapon, dreadnoughts only fought each other once, at the Battle of Jutland in May-June 1916. The battle was technically a draw with the Royal Navy suffering the heaviest losses, but it was, in fact, a strategic victory, since thereafter, the German fleet posed no threat to British waters. After Jutland, British battleships were largely used to enforce a blockade of Germany. Another successful ship was the battlecruiser (first called a fast armoured cruiser), which had the same armament as a battleship but, with lighter armoured plating, could reach a greater top speed of around 25 knots.

Aircraft were used by the Royal Flying Corps for reconnaissance of enemy positions, bombing, patrol duties, and as fighter planes against enemy airplanes and Zeppelin airships. Seaplanes, which had floats instead of wheels, could be flown from adapted ships which dropped them into the sea using a crane. The first purpose-built aircraft carrier – which planes took off from and landed back on again – was the Argo, but that ship was only active in the last months of the war. Biplanes were often unreliable mechanically and not always easy to control, which led to all sorts of unflattering nicknames for WWI aircraft, such as 'the dung hunter' and 'the flying coffin'.

WWI saw a bewildering array of aircraft built by a wide range of manufacturers. The Airco DH2 biplane combatted the threat of Germany's Fokker monoplanes with its improved manoeuvrability and rate of climb. The Airco DH4 was even better with a speed of 143 mph (230 km/h). With a place for the pilot and a gunner, the aircraft was armed with a Vickers machine gun and one or two Lewis guns. The DH4, in operation from March 1917, was designed as a fast bomber and could carry a bomb load of 460 lbs (208 kg) or, if used at sea, a number of depth charges. DH4s were also used by the American air force. Other successful aircraft included the Royal Aircraft Factory SE5 fighter and the Sopwith Camel. The Camel was, in fact, the most effective of all WWI fighter planes, accounting for 1,294 lost enemy aircraft. The famous fighter, over 5,400 of which were built, acquired its name because of the hump that covered the breeches of its twin Vickers machine guns.

Flying was a precarious business in these experimental days of air warfare, but it was, at least, the most glamorous of all the armed services. Many pilots became heroes back home, a status rarely enjoyed by infantry or seamen. Britain's top air aces were Major Edward Mannock with 73 hits, Lieutenant-Colonel William Bishop with 72 hits, and Lieutenant-Colonel Raymond Collishaw with 60 hits. This compares favourably to Germany's top air ace, Baron Manfred von Richthofen, the 'Red Baron', who had 80 hits before he himself was shot down and killed.