World War I
The Revolution of November was followed in December by an armistice and negotiations with Germany. At first, the Bolsheviks refused the German terms, but when German troops began marching across Ukraine unopposed, they acceded to the Treaty of Brest-Litovsk on 3 March 1918. The treaty ceded vast territories, including Finland, Estonia, Latvia, Lithuania, and parts of Poland and Ukraine to the Central Powers.
With the Russian Empire out of the war, Romania found itself alone on the Eastern Front and signed the Treaty of Bucharest with the Central Powers in May 1918. Under the terms of the treaty, Romania ceded territory to Austria-Hungary and Bulgaria and leased its oil reserves to Germany. However, the terms also included the Central Powers' recognition of the union of Bessarabia with Romania.

United States enters the war: The United States was a major supplier of war material to the Allies but remained neutral in 1914, in large part due to domestic opposition. The most significant factor in creating the support Wilson needed was the German submarine offensive, which not only cost American lives but paralysed trade as ships were reluctant to put to sea.
On 6 April 1917, Congress declared war on Germany as an "Associated Power" of the Allies. The US Navy sent a battleship group to Scapa Flow to join the Grand Fleet, and provided convoy escorts. In April 1917, the US Army had fewer than 300,000 men, including National Guard units, compared to British and French armies of 4.1 and 8.3 million respectively. The Selective Service Act of 1917 drafted 2.8 million men, though training and equipping such numbers was a huge logistical challenge. By June 1918, over 667,000 members of the American Expeditionary Forces (AEF) were transported to France, a figure which reached 2 million by the end of November.
Despite his conviction that Germany must be defeated, Wilson went to war to ensure the US played a leading role in shaping the peace, which meant preserving the AEF as a separate military force, rather than being absorbed into British or French units as his Allies wanted. He was strongly supported by AEF commander General John J. Pershing, a proponent of pre-1914 "open warfare" who considered the French and British emphasis on artillery misguided and incompatible with American "offensive spirit". Much to the frustration of his Allies, who had suffered heavy losses in 1917, he insisted on retaining control of American troops, and refused to commit them to the front line until able to operate as independent units. As a result, the first significant US involvement was the Meuse–Argonne offensive in late September 1918.

Nivelle Offensive (April–May 1917): In December 1916, Robert Nivelle replaced Pétain as commander of French armies on the Western Front and began planning a spring attack in Champagne, part of a joint Franco-British operation. Poor security meant German intelligence was well informed on tactics and timetables, but despite this, when the attack began on 16 April the French made substantial gains, before being brought to a halt by the newly built and extremely strong defences of the Hindenburg Line. Nivelle persisted with frontal assaults and, by 25 April, the French had suffered nearly 135,000 casualties, including 30,000 dead, most incurred in the first two days.
Concurrent British attacks at Arras were more successful, though ultimately of little strategic value. Operating as a separate unit for the first time, the Canadian Corps' capture of Vimy Ridge is viewed by many Canadians as a defining moment in creating a sense of national identity. Though Nivelle continued the offensive, on 3 May the 21st Division, which had been involved in some of the heaviest fighting at Verdun, refused orders to go into battle, initiating the French Army mutinies; within days, "collective indiscipline" had spread to 54 divisions, while over 20,000 deserted.

Sinai and Palestine campaign (1917–1918): In March and April 1917, at the First and Second Battles of Gaza, German and Ottoman forces stopped the advance of the Egyptian Expeditionary Force, which had begun in August 1916 at the Battle of Romani. At the end of October 1917, the Sinai and Palestine campaign resumed, when General Edmund Allenby's XXth Corps, XXI Corps and Desert Mounted Corps won the Battle of Beersheba. Two Ottoman armies were defeated a few weeks later at the Battle of Mughar Ridge and, early in December, Jerusalem had been captured following another Ottoman defeat at the Battle of Jerusalem. About this time, Friedrich Freiherr Kress von Kressenstein was relieved of his duties as the Eighth Army's commander, replaced by Djevad Pasha, and a few months later the commander of the Ottoman Army in Palestine, Erich von Falkenhayn, was replaced by Otto Liman von Sanders.
In early 1918, the front line was extended and the Jordan Valley was occupied, following the First Transjordan and the Second Transjordan attacks by British Empire forces in March and April 1918.

German offensive and Allied counter-offensive (March–November 1918): In December 1917, the Central Powers signed an armistice with Russia, thus freeing large numbers of German troops for use in the West. 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 in a final quick offensive. Ludendorff drew up plans (Operation Michael) for the 1918 offensive on the Western Front. The operation commenced on 21 March 1918, with an attack on British forces near Saint-Quentin. German forces achieved an unprecedented advance of 60 kilometres (37 mi). The initial offensive was a success; after heavy fighting, however, the offensive was halted. Lacking tanks or motorised artillery, the Germans were unable to consolidate their gains. The problems of re-supply were also exacerbated by increasing distances that now stretched over terrain that was shell-torn and often impassable to traffic.
Germany launched Operation Georgette against the northern English Channel ports. The Allies halted the drive after limited territorial gains by Germany. The German Army to the south then conducted Operations Blücher and Yorck, pushing broadly towards Paris. Germany launched Operation Marne (Second Battle of the Marne) on 15 July, in an attempt to encircle Reims. The resulting counter-attack, which started the Hundred Days Offensive on 8 August, led to a marked collapse in German morale.

Allied advance to the Hindenburg Line: By September, the Germans had fallen back to the Hindenburg Line. The Allies had advanced to the Hindenburg Line in the north and centre. German forces launched numerous counterattacks, but positions and outposts of the Line continued falling, with the BEF alone taking 30,441 prisoners in the last week of September. On 24 September, the Supreme Army Command informed the leaders in Berlin that armistice talks were inevitable.
The final assault on the Hindenburg Line began with the Meuse-Argonne offensive, launched by American and French troops on 26 September. Two days later the Belgians, French and British attacked around Ypres, and the day after the British at St Quentin in the centre of the line. The following week, cooperating American and French units broke through in Champagne at the Battle of Blanc Mont Ridge (3–27 October), forcing the Germans off the commanding heights, and closing towards the Belgian frontier. On 8 October, the Hindenburg Line was pierced by British and Dominion troops of the First and Third British Armies at the Second Battle of Cambrai.

Breakthrough of Macedonian front (September 1918): Allied forces started the Vardar offensive on 15 September at two key points: Dobro Pole and near Dojran Lake. In the Battle of Dobro Pole, the Serbian and French armies had success after a three-day-long battle with relatively small casualties, and subsequently made a breakthrough in the front, something which was rarely seen in World War I. After the front was broken, Allied forces started to liberate Serbia and reached Skopje at 29 September, after which Bulgaria signed an armistice with the Allies on 30 September.

Armistices and capitulations: The collapse of the Central Powers came swiftly. Bulgaria was the first to sign an armistice, the Armistice of Salonica on 29 September 1918. German Emperor Wilhelm II in a telegram to Bulgarian Tsar Ferdinand I described the situation thus: "Disgraceful! 62,000 Serbs decided the war!". On the same day, the German Supreme Army Command informed Kaiser Wilhelm II and the Imperial Chancellor Count Georg von Hertling, that the military situation facing Germany was hopeless.
On 24 October, the Italians began a push that rapidly recovered territory lost after the Battle of Caporetto. This culminated in the Battle of Vittorio Veneto, marking the end of the Austro-Hungarian Army as an effective fighting force. The offensive also triggered the disintegration of the Austro-Hungarian Empire. During the last week of October, declarations of independence were made in Budapest, Prague, and Zagreb.