The alliance system in Europe was one of the causes of the First World War (1914 to 18), although it did not make war inevitable. In the first decade of the 20th century, the Triple Entente powers of Great Britain, France, and Russia stood against the Triple Alliance of Germany, Austria-Hungary, and Italy (which remained neutral when war broke out but then joined the Entente powers in 1915). The terms of each state's commitment to its allies varied, but a general promise of aid was either stated or inferred. It was hoped that the system of alliances would create a balance of power, deter aggression, and maintain peace, but the alliances only added to a long list of other causes of WWI. In the end, signatures on paper became subservient to each nation's determination to either remain or become a global power, which resulted in leaders and generals competing for control of the territory and resources of other nations.

Britain was a well-established parliamentary democracy in the early 20th century. The monarchy, in the form of George V (reign 1910 to 1936), was little more than a figurehead of government. The British Empire enclosed some 400 million people in over 50 countries. Thanks to the Industrial Revolution and following centuries of imperialism around the globe, Britain's resources and military (especially its navy, the largest in the world) made it the most powerful and wealthiest country of all. Britain's position of dominance was being challenged by both the United States and Germany, the former in economic terms and the latter via an arms race. In 1904, the Entente Cordiale began, which saw closer diplomatic relations between Britain and France, removing conflicts of interest in Africa and Asia, but not covering mutual assistance in the case of a war in Europe. Germany, however, suspected a secret clause in the Entente Cordiale which did promise mutual military aid, such was the air of suspicion in European diplomacy in this period. In 1907, the Anglo-Russian Convention was signed, which diffused tensions over rival claims to Afghanistan, Tibet, and Persia (modern Iran). In 1912, Britain and France strengthened their alliance, with the former promising the formation of an expeditionary force to be sent to France if required.

France was a democratic republic headed by an elected president. Unlike in Britain, the French government contained a number of generals. The French Empire included North and West Africa and governed 60 million people. However, this empire was in decline, with a notable defeat to Prussia in 1870 (Prussia later became a part of Germany). France had once dominated world trade, but here, too, it was in decline by 1914. France formed an alliance with Russia in 1894, which promised mutual aid should they be attacked by Germany, Italy, or Austria-Hungary. In 1911, German interference in French Morocco, in an incident known as the Agadir or Moroccan Crisis (actually the second of two Moroccan Crises, the first being in 1905 to 6), further heightened the tensions between the European powers. Britain supported France, and Germany backed down but gained a slice of Congo as compensation. The incident meant that European powers now suspected Germany was intent on war sooner or later.

Russia was not a democracy but was ruled by Tsar Nicholas II (reign 1894 to 1917). The Tsar's autocratic rule had already been challenged by the failed Russian Revolution of 1905. The Russian Empire included around 160 million people, but there were many ethnic groups and nationalities, some of which wanted to break away from the empire and rule themselves. Russia was behind the more advanced nations in terms of the modernisation brought about by the Industrial Revolution. Around 80% of the population worked in agriculture, and their poverty meant most farmers were one bad harvest away from famine. In 1907, Britain, France, and Russia joined together in the Triple Entente. Henceforth, these countries and those states that later joined them were often called the Allies.

Germany was a product of the unification of various smaller states in 1871. The ruler was Kaiser Wilhelm II (reign 1888 to 1918), a constitutional monarch but a powerful one. Germany had only a small empire, limited to certain territories in East Africa and the Far East, but the Kaiser was ambitious to expand it, if for no other reason than prestige and to obtain the resources needed to feed Germany's booming economy. The Kaiser also talked of Germany being encircled by enemies: Britain, France, and Russia. Germany was industrialised and had the world's second-largest navy, the fastest-growing economy in Europe, and a booming population. Germany formed the Dual Alliance with Austria-Hungary in 1879, where each promised to defend the other if attacked by Russia. There was, too, a secret treaty with Russia that promised that state's neutrality if Germany ever attacked France.

The Austro-Hungarian Empire consisted of Austria and Hungary (joined since 1867), each of which had its own parliament and respective territories in Central Europe. The Habsburg emperor, Franz Joseph I of Austria (reign 1848 to 1916), was keen to expand his empire, especially in the Balkans. As with Russia, the empire enclosed many different ethnic groups and nationalities, a number of which actively worked for autonomy.

Italy had a constitutional monarchy headed by King Victor Emmanuel III (reign 1900 to 1946). Italy was only unified in 1861 but had won the Italo-Turkish War against the Ottoman Empire in 1912 and so gained control of Libya in North Africa. Italy joined with Germany and Austria-Hungary's Dual Alliance in 1882, and so it became the Triple Alliance. Italy, with a poorly equipped army, remained neutral when WWI broke out, but, hoping to gain territory from an obviously weak Austria-Hungary, whose early military escapades were disasters, joined the conflict in May 1915 on the side of the Triple Entente Powers.

Other states that joined WWI at various stages include Belgium, Japan, Montenegro, Serbia, Romania, Portugal, Greece, and the United States (which joined in 1917), all of which sided with the Entente Powers. The Ottoman Empire and Bulgaria joined the Triple Alliance Powers. Various colonies of the European powers in Africa, the Middle East, and Asia joined the conflict, too. Countries which remained neutral included Spain, Switzerland, the Netherlands, Norway, Sweden, and Denmark.

The Alliance system was not the only cause of WWI. After all, Britain was not obliged by treaty to join France in a European war but did just that. Conversely, Italy was bound to assist its allies, Germany and Austria-Hungary, but failed to do so when it declared its neutrality. In many ways, each nation's leader was trapped into pursuing actions whose consequences they did not necessarily intend, and those actions were ultimately driven by that nation's own long-term foreign policy aims. Britain was keen to maintain the geopolitical status quo, France was concentrated on its defence against German aggression, and Kaiser Wilhelm thought he could expand German territory at relatively little cost. Almost everyone thought that a war, if one did actually occur, would be tolerably short, perhaps over by Christmas 1914. Austria-Hungary was possibly the most guilty of the opportunists, hoping to profit from Germany's backing by expanding its territory in the Balkans with impunity despite Russia's declared interest in this Slavic region. In the end, each state's pursuit of what it considered its own national interests and the desire to control the arms, colonies, and the resources of others meant that one spark could light this volatile diplomatic powder keg of alliances. It is significant that Germany, France, Austria-Hungary, and Russia all had detailed military plans of attack ready long before any declarations of war were made.

The spark that set off WWI was the assassination of Archduke Franz Ferdinand (1863 to 1914), heir to the Habsburg throne, in Sarajevo in June 1914. The Balkans were a hotbed of nationalist movements, and the assassin was a Bosnian Serb nationalist. The Serbian government was promptly blamed for the assassination, and Franz Joseph gained the support of Germany to take over Serbia. This was despite Russia's threat that this would not be tolerated, a position backed by France. Germany threatened to declare war on Russia if it mobilised its troops. Germany realised that if it was to attack France, it would have to do so quickly before it also faced Russia in the east. To do so, the German generals put into action their Schlieffen Plan – to charge through Belgium and attack the French defensive lines from behind. Britain would not allow Belgium's neutrality to be so compromised. And so the domino effect of treaty obligations amongst the great powers was toppled into action by the events in Sarajevo. A flurry of communications flew between government offices across Europe. By 12 August, most of the major powers had all declared war on each other.

The first global war, the first wholly mechanised war, and the first arrival of total war, this dreadful conflict ended in 1918 and claimed the lives of over 9 million combatants. Victory for Britain, France, and Italy resulted in the destruction of four empires: the Austro-Hungarian, Russian, Ottoman, and German empires. The Treaty of Versailles and the formation of the League of Nations, it was hoped, would ensure a lasting peace, but another round of alliance-building could not prevent the Second World War (1939 to 45), an even bigger disaster than the first.