Of equal importance was the rampant spirit of nationalism, especially unsettling in the empire of Austria-Hungary and perhaps also in France. Nationalism had brought the unification of Germany by "blood and iron," and France, deprived of Alsace and Lorraine by the Franco-Prussian War of 1870-71, had been left with its own nationalistic cult seeking revenge against Germany. While French nationalists were hostile to Germany, which sought to maintain its gains by militarism and alliances, nationalism was creating violent tensions in the Austro-Hungarian Monarchy ; there the large Slavic national groups had grown increasingly restive, and Serbia as well as Russia fanned Slavic hopes for freedom and Pan-Slavism .

Imperialist rivalry had grown more intense with the "new imperialism" of the late 19th and early 20th cent. The great powers had come into conflict over spheres of influence in China and over territories in Africa, and the Eastern Question , created by the decline of the Ottoman Empire, had produced several disturbing controversies. Particularly unsettling was the policy of Germany. It embarked late but aggressively on colonial expansion under Emperor William II , came into conflict with France over Morocco , and seemed to threaten Great Britain by its rapid naval expansion.

These issues, imperialist and nationalist, resulted in a hardening of alliance systems in the Triple Alliance and Triple Entente and in a general armaments race. Nonetheless, a false optimism regarding peace prevailed almost until the onset of the war, an optimism stimulated by the long period during which major wars had been avoided, by the close dynastic ties and cultural intercourse in Europe, and by the advance of industrialization and economic prosperity. Many Europeans counted on the deterrent of war's destructiveness to preserve the peace.

The Austrian annexation (1908) of Bosnia and Hercegovina created an international crisis, but war was avoided. The Balkan Wars (1912-13) remained localized but increased Austria's concern for its territorial integrity, while the solidification of the Triple Alliance made Germany more yielding to the demands of Austria, now its one close ally. The assassination (June 28, 1914) of Archduke Francis Ferdinand at Sarajevo set in motion the diplomatic maneuvers that ended in war.

The Austrian military party, headed by Count Berchtold , won over the government to a punitive policy toward Serbia. On July 23, Serbia was given a nearly unacceptable ultimatum. With Russian support assured by Sergei Sazonov , Serbia accepted some of the terms but hedged on others and rejected those infringing upon its sovereignty. Austria-Hungary, supported by Germany, rejected the British proposal of Sir Edward Grey (later Lord Grey of Fallodon ) and declared war (July 28) on Serbia.

Russian mobilization precipitated a German ultimatum (July 31) that, when unanswered, was followed by a German declaration of war on Russia (Aug. 1). Convinced that France was about to attack its western frontier, Germany declared war (Aug. 3) on France and sent troops against France through Belgium and Luxembourg. Germany had hoped for British neutrality, but German violation of Belgian neutrality gave the British government the pretext and popular support necessary for entry into the war. In the following weeks Montenegro and Japan joined the Allies (Great Britain, France, Russia, Serbia, and Belgium) and the Ottoman Empire joined the Central Powers (Germany and Austria-Hungary). The war had become general. Whether it might have been avoided or localized and which persons and nations were most responsible for its outbreak are questions still debated by historians.

The German strategy, planned by Alfred von Schlieffen, called for an attack on the weak left flank of the French army by a massive German force approaching through Belgium, while maintaining a defensive stance toward Russia, whose army, Schlieffen assumed, would require six weeks to mobilize. By that time, Germany would have captured France and would be ready to meet the forces on the Eastern Front. The Schlieffen plan was weakened from the start when the German commander Helmuth von Moltke detached forces from the all-important German right wing, which was supposed to smash through Belgium, in order to reinforce the left wing in Alsace-Lorraine. Nevertheless, the Germans quickly occupied most of Belgium and advanced on Paris.

In Sept., 1914, the first battle of the Marne took place. For reasons still disputed, a general German retreat was ordered after the battle, and the Germans entrenched themselves behind the Aisne River. The Germans then advanced toward the Channel ports but were stopped in the first battle of Ypres; grueling trench warfare ensued along the entire Western Front. Over the next three years the battle line remained virtually stationary. It ran, approximately, from Ostend past Armentières, Douai, Saint-Quentin, Reims, Verdun, and Saint-Mihiel to Lunéville.

Meanwhile, on the Eastern Front, the Russians invaded East Prussia but were decisively defeated (Aug.-Sept., 1914) by the Germans under generals Hindenburg, Ludendorff, and Mackensen at Tannenberg and the Masurian Lakes. The Germans advanced on Warsaw, but farther south a Russian offensive drove back the Austrians. However, by the autumn of 1915 combined Austro-German efforts had driven the Russians out of most of Poland and were holding a line extending from Riga to Chernovtsy (Chernivtsi). The Russians counterattacked in 1916 in a powerful drive directed by General Brusilov , but by the year's end the offensive had collapsed, after costing Russia many thousands of lives. Soon afterward the Russian Revolution eliminated Russia as an effective participant in the war. Although the Austro-Hungarians were unsuccessful in their attacks on Serbia and Montenegro in the first year of the war, these two countries were overrun in 1915 by the Bulgarians (who had joined the Central Powers in Oct., 1915) and by Austro-German forces.

Another blow to the Allied cause was the failure in 1915 of the Gallipoli campaign , an attempt to force Turkey out of the war and to open a supply route to S Russia. The Allies, however, won a diplomatic battle when Italy, after renouncing its partnership in the Triple Alliance and after being promised vast territorial gains, entered the war on the Allied side in May, 1915. Fighting between Austria and Italy along the Isonzo River was inconclusive until late 1917, when the rout of the Italians at Caporetto made Italy a liability rather than an asset to the Allies.

Except for the conquest of most of Germany's overseas colonies by the British and Japanese, the year 1916 opened with a dark outlook for the Allies. The stalemate on the Western Front had not been affected in 1915 by the second battle of Ypres, in which the Germans used poison gas for the first time on the Western Front, nor by the French offensive in Artoisin which a slight advance of the French under Henri Pétain was paid for with heavy lossesnor by the offensive of Marshal Joffre in Champagne, nor by the British advance toward Lens and Loos.

In Feb., 1916, the Germans tried to break the deadlock by mounting a massive assault on Verdun. The French, rallying with the cry, "They shall not pass!" held fast despite enormous losses, and in July the British and French took the offensive along the Somme River where tanks were used for the first time by the British. By November they had gained a few thousand yards and lost thousands of men. By December, a French counteroffensive at Verdun had restored the approximate positions of Jan., 1916.

Despite signs of exhaustion on both sides, the war went on, drawing ever more nations into the maelstrom. Portugal and Romania joined the Allies in 1916; Greece, involved in the war by the Allied Salonica campaigns on its soil, declared war on the Central Powers in 1917.

The neutrality of the United States had been seriously imperiled after the sinking of the Lusitania (1915). At the end of 1916, Germany, whose surface fleet had been bottled up since the indecisive battle of Jutland, announced that it would begin unrestricted submarine warfare in an effort to break British control of the seas. In protest the United States broke off relations with Germany (Feb., 1917), and on Apr. 6 it entered the war. American participation meant that the Allies now had at their command almost unlimited industrial and manpower resources, which were to be decisive in winning the war. It also served from the start to lift Allied morale, and the insistence of President Woodrow Wilson on a "war to make the world safe for democracy" was to weaken the Central Powers by encouraging revolutionary groups at home.

The war on the Western Front continued to be bloody and stalemated. But in the Middle East the British, who had stopped a Turkish drive on the Suez Canal, proceeded to destroy the Ottoman Empire; T. E. Lawrence stirred the Arabs to revolt, Baghdad fell (Mar., 1917), and Field Marshal Allenby took Jerusalem (Dec., 1917). The first troops of the American Expeditionary Forces (AEF), commanded by General Pershing , landed in France in June, 1917, and were rushed to the Château-Thierry area to help stem a new German offensive.

A unified Allied command in the West was created in Apr., 1918. It was headed by Marshal Foch , but under him the national commanders (Sir Douglas Haig for Britain, King Albert I for Belgium, and General Pershing for the United States) retained considerable authority. The Central Powers, however, had gained new strength through the Treaty of Brest-Litovsk (Mar., 1918) with Russia. The resources of Ukraine seemed at their disposal, enabling them to balance to some extent the effects of the Allied blockade; most important, their forces could now be concentrated on the Western Front.

The critical German counteroffensive, known as the second battle of the Marne, was stopped just short of Paris (July-Aug., 1918). At this point Foch ordered a general counterattack that soon pushed the Germans back to their initial line (the so-called Hindenburg Line). The Allied push continued, with the British advancing in the north and the Americans attacking through the Argonne region of France. While the Germans were thus losing their forces on the Western Front, Bulgaria, invaded by the Allies under General Franchet d'Esperey , capitulated on Sept. 30, and Turkey concluded an armistice on Oct. 30. Austria-Hungary, in the process of disintegration, surrendered on Nov. 4 after the Italian victory at Vittorio Veneto.

German resources were exhausted and German morale had collapsed. President Wilson's Fourteen Points were accepted by the new German chancellor, Maximilian, prince of Baden , as the basis of peace negotiations, but it was only after revolution had broken out in Germany that the armistice was at last signed (Nov. 11) at Compiègne. Germany was to evacuate its troops immediately from all territory W of the Rhine, and the Treaty of Brest-Litovsk was declared void. The war ended without a single truly decisive battle having been fought, and Germany lost the war while its troops were still occupying territory from France to Crimea. This paradox became important in subsequent German history, when nationalists and militarists sought to blame the defeat on traitors on the home front rather than on the utter exhaustion of the German war machine and war economy.

World War I and the resulting peace treaties radically changed the face of Europe and precipitated political, social, and economic changes. By the Treaty of Versailles Germany was forced to acknowledge guilt for the war. Later, prompted by the Bolshevik publication of the secret diplomacy of the czarist Russian government, the warring powers gradually released their own state papers, and the long historical debate on war guilt began. It has with some justice been claimed that the conditions of the peace treaties were partially responsible for World War II . Yet when World War I ended, the immense suffering it had caused gave rise to a general revulsion to any kind of war, and a large part of mankind placed its hopes in the newly created League of Nations.

To calculate the total losses caused by the war is impossible. About 10 million dead and 20 million wounded is a conservative estimate. Starvation and epidemics raised the total in the immediate postwar years. Warfare itself had been revolutionized by the conflict.