World War I, Spring Offensive: German General Erich Ludendorff drew up plans (codenamed Operation Michael) for the 1918 offensive on the Western Front. The Spring Offensive sought to divide the British and French forces with a series of feints and advances. The German leadership hoped to strike a decisive blow before significant U.S. forces arrived. The operation commenced on 21 March 1918 with an attack on British forces near Amiens. German forces achieved an unprecedented advance of 60 kilometres (40 miles).

British and French trenches were penetrated using novel infiltration tactics, also named Hutier tactics, after General Oskar von Hutier. Previously, attacks had been characterized by long artillery bombardments and massed assaults. However, in the Spring Offensive of 1918, Ludendorff used artillery only briefly and infiltrated small groups of infantry at weak points. They attacked command and logistics areas and bypassed points of serious resistance. More heavily armed infantry then destroyed these isolated positions. German success relied greatly on the element of surprise.

The front moved to within 120 kilometers (75 mi) of Paris. Three heavy Krupp railway guns fired 183 shells on the capital, causing many Parisians to flee. The initial offensive was so successful that Kaiser Wilhelm II declared 24 March a national holiday. Many Germans thought victory was near. After heavy fighting, however, the offensive was halted. Lacking tanks or motorised artillery, the Germans were unable to consolidate their gains. This situation was not helped by the supply lines now being stretched as a result of their advance. The sudden stop was also a result of the four Australian Imperial Force (AIF) divisions that were "rushed" down, thus doing what no other army had done and stopping the German advance in its tracks. During that time the first Australian division was hurriedly sent north again to stop the second German breakthrough.

General Foch pressed to use the arriving American troops as individual replacements. Pershing sought instead to field American units as an independent force. These units were assigned to the depleted French and British Empire commands on 28 March. A Supreme War Council of Allied forces was created at the Doullens Conference on 5 November 1917. General Foch was appointed as supreme commander of the allied forces. Haig, Petain and Pershing retained tactical control of their respective armies; Foch assumed a coordinating role, rather than a directing role and the British, French and U.S. commands operated largely independently.

Following Operation Michael, Germany launched Operation Georgette against the northern English channel ports. The Allies halted the drive with limited territorial gains for Germany. The German Army to the south then conducted Operations Blücher and Yorck, broadly towards Paris. Operation Marne was launched on 15 July, attempting to encircle Reims and beginning the Second Battle of the Marne. The resulting counterattack, starting the Hundred Days Offensive, marked their first successful Allied offensive of the war.

By 20 July the Germans were back across the Marne at their Kaiserschlacht starting lines, having achieved nothing. Following this last phase of the war in the West, the German Army never again regained the initiative. German casualties between March and April 1918 were 270,000, including many highly trained stormtroops.

Meanwhile, Germany was falling apart at home. Anti-war marches became frequent and morale in the army fell. Industrial output was 53 percent of 1913 levels.

Non-Intervention

The United States originally pursued a policy of non-intervention, avoiding conflict while trying to broker a peace. When a German U-boat sank the British liner Lusitania in 1915, with 128 Americans aboard, U.S. President Woodrow Wilson vowed, "America is too proud to fight" and demanded an end to attacks on passenger ships. Germany complied. Wilson unsuccessfully tried to mediate a settlement. He repeatedly warned the U.S. would not tolerate unrestricted submarine warfare, in violation of international law and U.S. ideas of human rights. Wilson was under pressure from former president Theodore Roosevelt, who denounced German acts as "piracy". Wilson's desire to have a seat at negotiations at war's end to advance the League of Nations also played a role. Wilson's Secretary of State, William Jennings Bryan, resigned in protest at what he felt was the President's decidedly warmongering diplomacy. Other factors contributing to the U.S. entry into the war include the suspected German sabotage of Black Tom in Jersey City, New Jersey, and the Kingsland Explosion in what is now Lyndhurst, New Jersey.

Making the case

In January 1917, after the Navy pressured the Kaiser, Germany resumed unrestricted submarine warfare. Britain's secret Royal Navy cryptanalytic group, Room 40, had broken the German diplomatic code. They intercepted a proposal from Berlin (the Zimmermann Telegram) to Mexico to join the war as Germany's ally against the United States, should the U.S. join. The proposal suggested that if the U.S. were to enter the war then Mexico should declare war against the United States and enlist Japan as an ally. This would prevent the United States from joining the Allies and deploying troops to Europe, and would give Germany more time for their unrestricted submarine warfare program to strangle Britain's vital war supplies. In return, the Germans would promise Mexico support in reclaiming the territories of Texas, New Mexico, and Arizona that Mexico lost during the Mexican-American War 70 years earlier.

U.S. declaration of war on Germany

After the British revealed the telegram to the United States, President Wilson, who had won reelection on his keeping the country out of the war, released the captured telegram as a way of building support for U.S. entry into the war. He had previously claimed neutrality, while calling for the arming of U.S. merchant ships delivering munitions to combatant Britain and quietly supporting the British blockading of German ports and mining of international waters, preventing the shipment of food from America and elsewhere to combatant Germany. After submarines sank seven U.S. merchant ships and the publication of the Zimmerman telegram, Wilson called for war on Germany, which the U.S. Congress declared on 6 April 1917.

Crucial to U.S. participation was the sweeping domestic propaganda campaign executed by the Committee on Public Information, overseen by George Creel. The campaign consisted of tens of thousands of government-selected community leaders giving brief carefully scripted pro-war speeches at thousands of public gatherings. Along with other branches of government and private vigilante groups like the American Protective League, it also included the general repression and harassment of people either opposed to American entry into the war or of German heritage. Other forms of propaganda included newsreels, photos, large-print posters, magazine and newspaper articles, and so forth. Additionally, during World War I, Woodrow Wilson placed a great importance on children, especially the Boy Scouts of America, asking them to encourage war support and educate the public about the importance of the war. They helped distribute these war pamphlets, helped sell war bonds, and helped to drive nationalism and support for the war.

First active U.S. participation in World War I

The United States was never formally a member of the Allies but became a self-styled "Associated Power". The United States had a small army, but, after the passage of the Selective Service Act, it drafted 2.8 million men and by summer 1918 was sending 10,000 fresh soldiers to France every day. In 1917, the U.S. Congress gave U.S. citizenship to Puerto Ricans when they were drafted to participate in World War I, as part of the Jones Act. Germany had miscalculated, believing it would be many more months before they would arrive and that the arrival could be stopped by U-boats. The United States Navy sent a battleship group to Scapa Flow to join with the British Grand Fleet, destroyers to Queenstown, Ireland and submarines to help guard convoys. Several regiments of U.S. Marines were also dispatched to France. The British and French wanted U.S. units used to reinforce their troops already on the battle lines and not waste scarce shipping on bringing over supplies. The U.S. rejected the first proposition and accepted the second. General John J. Pershing, American Expeditionary Force (AEF) commander, refused to break up U.S. units to be used as reinforcements for British Empire and French units. As an exception, he did allow African-American combat regiments to be used in French divisions. The Harlem Hellfighters fought as part of the French 16th Division, earning a unit Croix de Guerre for their actions at Chateau-Thierry, Belleau Wood and Sechault. AEF doctrine called for the use of frontal assaults, which had long since been discarded by British Empire and French commanders because of the large loss of life.

Events of 1917 proved decisive in ending the war, although their effects were not fully felt until 1918.

The British naval blockade began to have a serious impact on Germany. In response, in February 1917, the German General Staff convinced Chancellor Theobald von Bethmann Hollweg to declare unrestricted submarine warfare, with the goal of starving Britain out of the war. Tonnage sunk rose above 500,000 tons per month from February to July. It peaked at 860,000 tons in April. After July, the newly re-introduced convoy system became extremely effective in reducing the U-boat threat. Britain was safe from starvation and German industrial output fell.

On 3 May 1917, during the Nivelle Offensive, the weary French 2nd Colonial Division, veterans of the Battle of Verdun, refused their orders, arriving drunk and without their weapons. Their officers lacked the means to punish an entire division, and harsh measures were not immediately implemented. Then, mutinies afflicted an additional 54 French divisions and saw 20,000 men desert. The other Allied forces attacked but sustained tremendous casualties. However, appeals to patriotism and duty, as well as mass arrests and trials, encouraged the soldiers to return to defend their trenches, although the French soldiers refused to participate in further offensive action. Robert Nivelle was removed from command by 15 May, replaced by General Philippe Pétain, who suspended bloody large-scale attacks.

The victory of Austria–Hungary and Germany at the Battle of Caporetto led the Allies at the Rapallo Conference to form the Supreme War Council to coordinate planning. Previously, British and French armies had operated under separate commands.

In December, the Central Powers signed an armistice with Russia. This released troops for use in the west. Ironically, German troop transfers could have been greater if their territorial acquisitions had not been so dramatic. With German reinforcements and new American troops pouring in, the outcome was to be decided on the Western front. The Central Powers knew that they could not win a protracted war, but they held high hopes for success based on a final quick offensive. Furthermore, the leaders of the Central Powers and the Allies became increasingly fearful of social unrest and revolution in Europe. Thus, both sides urgently sought a decisive victory.

In December 1916, after ten brutal months of the Battle of Verdun, the Germans attempted to negotiate a peace with the Allies, effectively declaring themselves the victors. Soon after, U.S. President Wilson attempted to intervene as a peacemaker, asking in a note for both sides to state their demands. Lloyd George's War Cabinet considered the German offer as a ploy to create divisions amongst the Allies. After initial outrage and much deliberation, they took Wilson's note as a separate effort, signalling that the U.S. was on the verge of entering the war against Germany following the "submarine outrages". While the Allies debated a response to Wilson's offer, the Germans chose to rebuff it in favour of "a direct exchange of views". Learning of the German response, the Allied governments were free to make clear demands in their response of 14 January. They sought restoration of damages, the evacuation of occupied territories, reparations for France, Russia and Roumania, and a recognition of the principle of nationalities. This included the liberation of Italians, Slavs, Roumanians, Czecho-Slovaks, and the creation of a "free and united Poland". On the question of security, the Allies sought guarantees that would prevent or limit future wars, complete with sanctions, as a condition of any peace settlement.

Initial actions

While the Western Front had reached stalemate, the war continued in East Europe. Initial Russian plans called for simultaneous invasions of Austrian Galicia and German East Prussia. Although Russia's initial advance into Galicia was largely successful, they were driven back from East Prussia by Hindenburg and Ludendorff at Tannenberg and the Masurian Lakes in August and September 1914. Russia's less developed industrial base and ineffective military leadership was instrumental in the events that unfolded. By the spring of 1915, the Russians had retreated into Galicia, and in May the Central Powers achieved a remarkable breakthrough on Poland's southern frontiers. On 5 August they captured Warsaw and forced the Russians to withdraw from Poland.

Russian Revolution

Despite the success of the June 1916 Brusilov offensive in eastern Galicia, dissatisfaction with the Russian government's conduct of the war grew. The success was undermined by the reluctance of other generals to commit their forces to support the victory. Allied and Russian forces were revived only temporarily with Romania's entry into the war on 27 August. German forces came to the aid of embattled Austro-Hungarian units in Transylvania and Bucharest fell to the Central Powers on 6 December. Meanwhile, unrest grew in Russia, as the Tsar remained at the front. Empress Alexandra's increasingly incompetent rule drew protests and resulted in the murder of her favourite, Rasputin, at the end of 1916.

In March 1917, demonstrations in Petrograd culminated in the abdication of Tsar Nicholas II and the appointment of a weak Provisional Government which shared power with the Petrograd Soviet socialists. This arrangement led to confusion and chaos both at the front and at home. The army became increasingly ineffective.

The war and the government became increasingly unpopular. Discontent led to a rise in popularity of the Bolshevik party, led by Vladimir Lenin. He promised to pull Russia out of the war and was able to gain power. The triumph of the Bolsheviks in November was followed in December by an armistice and negotiations with Germany. At first the Bolsheviks refused the German terms, but when Germany resumed the war and marched across Ukraine with impunity, the new government acceded to the Treaty of Brest-Litovsk on 3 March 1918. It took Russia out of the war and ceded vast territories, including Finland, the Baltic provinces, parts of Poland and Ukraine to the Central Powers. The manpower required for German occupation of former Russian territory may have contributed to the failure of the Spring Offensive, however, and secured relatively little food or other materiel

With the adoption of the Treaty of Brest-Litovsk, the Entente no longer existed. The Allied powers led a small-scale invasion of Russia, partly to stop Germany from exploiting Russian resources and, to a lesser extent, to support the "Whites" (as opposed to "Reds") in the Russian Civil War. Allied troops landed in Archangel and in Vladivostok.

The war began with an unprecedented outpouring of loyalty and goodwill towards the United Kingdom from within the mainstream political leadership, contrary to initial British fears of an Indian revolt. The Indian Army in fact outnumbered the British Army at the beginning of the war. India under British rule contributed greatly to the British war effort by providing men and resources. This was done by the Indian Congress in hope of achieving self-government as India was very much under the control of the British. The United Kingdom disappointed the Indians by not providing self-governance, leading to the Gandhian Era in Indian history. About 1.3 million Indian soldiers and labourers served in Europe, Africa, and the Middle East, while both the Indian government and the princes sent large supplies of food, money, and ammunition. In all 140,000 men served on the Western Front and nearly 700,000 in the Middle East. Casualties of Indian soldiers totalled 47,746 killed and 65,126 wounded during World War I.

Romania had been allied with the Central Powers since 1882. When the war began, however, it declared its neutrality, arguing that because Austria-Hungary had itself declared war on Serbia, Romania was under no obligation to join the war. When the Entente Powers promised Romania large territories of eastern Hungary (Transylvania and Banat), that had a large Romanian population, in exchange for Romania’s declaring war on the Central Powers, the Romanian government renounced its neutrality, and on 27 August 1916 the Romanian army launched an attack against Austria-Hungary. The Romanian offensive was initially successful, pushing back the Austro-Hungarian troops in Transylvania, but a counter attack by the forces of the Central Powers defeated the Romanian army and as a result of the Battle of Bucharest the Central Powers occupied Bucharest on 6 December 1916. Fighting in Moldova continued in 1917 until an armistice was signed between the Central Powers and Romania on 9 December 1917.

In January 1918, Russia, allied to Romania, had to withdraw its troops from the Romanian front and Romanian forces established control over Bessarabia. Although a treaty was signed by the Romanian and the Bolshevik Russian government following talks between 5-9 March 1918 on the withdrawal of Romanian forces from Bessarabia within two months, on 27 March 1918 Romania attached Bessarabia to its territory, formally based on a resolution passed by the local assembly of the territory on the unification with Romania.

Romania officially made peace with the Central Powers signing the Treaty of Bucharest on 7 May 1918. Under that treaty Romania was obliged to cease war with the Central Powers. Romania made small territorial concessions for Austria-Hungary, ceding control of some passes in the Carpathian mountains and granted oil concessions for Germany. On the other hand, the Central Powers recognized the sovereignty of Romania over Bessarabia. The treaty was renounced in October 1918 by the Alexandru Marghiloman government and Romania nominally re-entered the war on 10 November 1918. The next day, the Treaty of Bucharest was nullified by the terms of the Armistice of Compiègne. Total Romanian deaths from 1914 to 1918, military and civilian, within contemporary borders, were estimated at 748,000.