The First World War saw the Entente Powers, led by France, Russia, the British Empire, and later Italy (from 1915) and the United States (from 1917), defeat the Central Powers, led by the German, Austro-Hungarian, Bulgarian and Ottoman Empires. Russia withdrew from the war after the revolution in 1917.
On the battlefields of northern Europe, the British Empire fought against the enemy and the elements along a system of trenches that stretched over 600 kilometres from the Channel to the Swiss frontier.
The attritional struggle here transformed the landscape of northern France and Belgium into a lunar mud-scape of shell craters, corpses, abandoned equipment and wire entanglements. Commanders faced the tactical problem of getting troops safely across this fire-swept terrain to penetrate the enemy’s defences.
For most of the war, the front here was locked in stalemate, despite a series of costly offensives. This bitter struggle to overcome the deadlock defined the conflict for many involved. It also decided its outcome, with an Allied breakthrough finally defeating the Germans in 1918.
The Middle East was the largest theatre of war, and saw the British Empire and France fight against the Ottoman Empire in Mesopotamia (now Iraq), Gallipoli, Sinai and Palestine, and Persia.
On the whole, the campaigns of the Middle East were more mobile, as each side attempted to seize and defend key objectives for control over Middle Eastern territories.
Some troops had to endure the unforgiving desert environment, where disease was rife and temperatures could soar or plummet. A more difficult theatre in which to fight would be hard to imagine as it also created logistical and tactical problems for the armies.
Fighting also raged in the Balkans, where the British and French helped their Serbian ally engage the Bulgars. In 1917 the two allies also despatched troops to assist the Italians in their fight against the Austrians.
Hostilities also took place in Africa, where the British seized enemy colonies before fighting a long war against German guerrillas. In Eastern Europe, the Russians battled the Germans and Austrians, while simultaneously fighting the Ottomans in the Caucasus.
For the first time, battles took place in the air as well as on land. Aircraft, a new technology at the time, opened up a whole new theatre of war.
The British Army established a corps of men to fly these new machines - the Royal Flying Corps. But it was only through experience that they learnt how important a role they could play.
Control of the skies had an impact on the war on the ground. Troops could gather intelligence on enemy positions and direct artillery. But as the war unfolded, so too did the tactics of aerial warfare, including dog fighting, ground strafing and strategic bombing.
Fierce fighting also took place at sea. Naval offensives blurred the lines of combat. Attacks by surface vessels and submarines, alongside blockades of ports, affected both military personnel and civilians.
The war brought an end to a world order that had existed since the end of the Napoleonic Wars (1803-15). It caused the disintegration of four empires - the Austro-Hungarian, German, Ottoman and Russian - and was a key factor in the outbreak of the Second World War (1939-45).