France entered World War I when Germany declared war on 3 August 1914.

World War I largely arose from a conflict between two alliances: the Triple Alliance (Germany, Austria-Hungary, and Italy) and the Triple Entente (France, Russia, and Britain). France had had a military alliance with Russia since 1894, designed primarily to neutralize the German threat to both countries. Germany had a military alliance with Austria-Hungary.

In June 1914, Archduke Franz Ferdinand, the heir to the Austro-Hungarian throne, was assassinated. The government of Austria-Hungary entered into a conflict with Serbia due to the increasing unrest among ethnic Slavs. Germany secretly gave Austria-Hungary a blank check, promising to support it militarily no matter what it decided. Both countries wanted a localized war.

Russia decided to intervene to protect Serbia due to its interest in the Balkan region and its desire to gain an advantage over Austria-Hungary. The Tsar had the support of the President of France, who otherwise was hardly involved. Russia mobilized its army against Austria-Hungary. France mobilized its army. Germany declared war on Russia and France, and invaded France through Belgium. Britain had an understanding and military and naval planning agreements with France, but no formal treaty obligations. London felt British interest required a defence of France. Britain did have a treaty obligation toward Belgium, and that was used as the official reason Britain declared war on Germany. Japan, allied with Britain, was not obligated to go to war but did so to gain spoils. Turkey joined the Central Powers. Italy, instead of joining Germany and Austria-Hungary with whom it had treaties, entered the war on the side of the Allies in 1915. The United States tried unsuccessfully to broker peace negotiations, and entered the war on the Allied side in April 1917. After very heavy losses on both sides, the Allies were decisively victorious, and divided assets, such as the German colonies and much of the territory of the Ottoman Empire. The Austro-Hungarian, German, Russian and Ottoman Empires disintegrated.

By the late 1880s, Bismarck's League of the Three Emperors was in disarray; although Germany remained closely allied to Austria-Hungary, there was growing friction between Russia and Austria-Hungary over the Balkans. Angered by Austria's role in the Treaty of Berlin (1878), which forced Russia to withdraw from Bulgaria, Tsar Alexander III refused to renew the treaty in 1887. Bismarck, in the hope of making the Tsar more amenable to his wishes, had forbidden German banks to lend money to Russia. French bankers quickly replaced the Germans in financing Russia, and helped to speed Russian industrialization. The Russians had borrowed around 500 million francs by 1888. Bismarck signed a Reinsurance Treaty with Russia in 1887, but after Bismarck's fall from power in 1890 Kaiser William II refused Russia's request to renew it.

The advantage of a Franco-Russian alliance was clear to all Frenchmen: France would not be alone against Germany, for it promised a two-front war. Formal visits were exchanged between the two powers in 1890 and 1891, and the Russian Tsar saluted the French national anthem, La Marseillaise. The Franco-Russian alliance was announced in 1894. This diplomatic coup was followed by a secret agreement with Italy, allowing the Italians a free hand to expand in Tripoli (modern Libya, then still under Turkish rule). In return, Italy promised she would remain non-belligerent against France in any future war. Meanwhile, as Britain became increasingly anxious over the German naval buildup and industrial rivalry, agreement with France became increasingly attractive.

France competed with Britain, and to a lesser extent with Italy, for control of Africa. There was constant friction between Britain and France over borders between their respective African colonies (see the Fashoda Incident). The French Foreign Minister Théophile Delcassé was aware that France could not progress if she was in conflict with Germany in Europe and Britain in Africa, and so recalled Captain Marchand's expeditionary force from Fashoda, despite popular protests. This paved the way for Britain joining France in World War I.

Edward VII's visit to Paris in 1903 stilled anti-British feeling in France, and prepared the way for the Entente Cordiale. Initially however, a colonial agreement against the Kaiser's aggressive foreign policy deepened rather than destroyed the bond between the two countries. The Moroccan Crises of 1905 and 1911 encouraged both countries to embark on a series of secret military negotiations in the case of war with Germany. However, British Foreign Minister Edward Grey realized the risk that small conflicts between Paris and Berlin could escalate out of control. Working with little supervision from the British Prime Minister or Cabinet, Grey deliberately played a mediating role, trying to calm both sides and thereby maintain a peaceful balance of power. He refused to make permanent commitments to France. He approved military staff talks with France in 1905, thereby suggesting, but not promising, that if war broke out Britain would favor France over Germany. In 1911, when there was a second Franco-German clash over Morocco, Grey tried to moderate the French while supporting Germany in its demand for compensation. There was little risk that Britain would have conflicts with anyone leading to war. The Royal Navy remained dominant in world affairs, and remained a high spending priority for the British government. The British Army was small, although plans to send an expeditionary force to France had been developed since the Haldane Reforms. From 1907 through 1914, the French and British armies collaborated on highly detailed plans for mobilizing a British Expeditionary Force of 100,000 combat troops to be very quickly moved to France, and sent to the front in less than two weeks. Grey insisted that world peace was in the best interests of Britain and the British Empire.