An American doughboy, circa 1918.

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 reintroduced convoy system became extremely effective in neutralizing the U-boat threat. Britain was safe from starvation and German industrial output fell.

The victory of Austria-Hungary and Germany at the Battle of Caporetto led the Allied at the Rapallo Conference to form the Supreme Allied 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 final 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 a 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.

The United States originally pursued a policy of isolationism, avoiding conflict while trying to broker a peace. This resulted in increased tensions with Berlin and London. When a German U-boat sank the British liner Lusitania in 1915, with 128 Americans aboard, U.S. President Woodrow Wilson vowed, "America was 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 significant role. Wilson's Secretary of State, William Jennings Bryan, resigned in protest of the President's decidedly warmongering diplomacy. Other factors contributing to the U.S. entry into the war include the suspected German sabotage of both Black Tom in Jersey City, New Jersey, and the Kingsland Explosion in what is now Lyndhurst, New Jersey.

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 famous, but often overlooked, Zimmermann Telegram) to Mexico to join the war as Germany's ally against the United States, should the U.S. join. The proposal suggested, if the U.S. were to enter the war, 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 Texas, New Mexico, and Arizona.

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 massive domestic propaganda campaign executed by the Committee on Public Information overseen by George Creel. The campaign included 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.

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 it drafted four million men and by summer 1918 was sending 10,000 fresh soldiers to France every day. In 1917, the U.S. Congress imposed U.S. citizenship to Puerto Ricans as part of the Jones Act, when they were drafted to participate in World War I. Germany had miscalculated 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 (though he did allow African-American combat units to be used by the French). 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.

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. Before the offensive began, Ludendorff left the elite Eighth Army in Russia and sending over only a small portion of the German forces to the west.

Operation Michael opened on 21 March 1918. British forces were attacked near Amiens. Ludendorff wanted to split the British and French armies. German forces achieved an unprecedented advance of 60 kilometers (40 miles).

British and French trenches were penetrated using novel infiltration tactics, also named Hutier tactics, after General Oskar von Hutier. Attacks had been characterised by long artillery bombardments and massed assaults. However, in the Spring Offensive, the German Army 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. The sudden stop was also a result of the four AIF (Australian Imperial Forces) divisions that were "rushed" down, thus doing what no other army had done and stopping the German advance in its tracks. While during that time the first Australian division was hurriedly sent north again to stop the second German break through.

American divisions, which Pershing had sought to field as an independent force, 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 Allied counterattack marked their first successful offensive of the war.

By 20 July, the Germans were back 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 stormtroopers.

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

In 1918, the internationally recognized Democratic Republic of Armenia and Democratic Republic of Georgia bordering the Ottoman Empire were established, as well as the unrecognized Centrocaspian Dictatorship and South West Caucasian Republic.

In 1918, the Dashnaks of Armenian national liberation movement declared the Democratic Republic of Armenia (DRA) through the Armenian Congress of Eastern Armenians (unified form of Armenian National Councils) after the dissolution of Transcaucasian Democratic Federative Republic. Tovmas Nazarbekian become the first Commander-in-chief of DRA. Enver Pasha ordered the creation of a new army to be named the Army of Islam. He ordered the Army of Islam into DRA, with the goal of taking Baku on the Caspian Sea. This new offensive was strongly opposed by the Germans. In early May 1918, the Ottoman army attacked the newly declared DRA. Although the Armenians managed to inflict one defeat on the Ottomans at the Battle of Sardarapat, the Ottoman army won a later battle and scattered the Armenian army. The Republic of Armenia was forced to sign the Treaty of Batum in June 1918.

The Allied counteroffensive, known as the Hundred Days Offensive, began on 8 August 1918. The Battle of Amiens developed with III Corps Fourth British Army on the left, the First French Army on the right, and the Australian and Canadian Corps spearheading the offensive in the centre. It involved 414 tanks of the Mark IV and Mark V type, and 120,000 men. They advanced 12 kilometers (7 miles) into German-held territory in just seven hours. Erich Ludendorff referred to this day as the "Black Day of the German army".

The Australian-Canadian spearhead at Amiens, a battle that was the beginning of Germany’s downfall, helped pull the British Armies to the north and the French Armies to the south forward starting the momentum that eventually forced the German Armies back along the western front and into the Hindenburg Line. While German resistance on the British Fourth Army front at Amiens stiffened, after an advance as far as 14 miles (23 km) and brought the battle there to an end, the French Third Army lengthened the Amiens front on August 10, when it was thrown in on the right of the French First Army, and advanced four miles (6 km) liberating Lassigny in fighting which lasted until the 16th. South of the French Third Army General Mangin (The Butcher) drove his French Tenth Army forward at Soissons on August 20 to capture eight thousand prisoners, two hundred guns and the Aisne heights overlooking and menacing the German position north of the Vesle. Another "Black day" as described by Ludendorff.

Meanwhile General Byng of the Third British Army, reporting that the enemy on his front was thinning in a limited withdrawal, was ordered to attack with 200 tanks toward Bapaume opening what is known as the Battle of Albert with the specific orders of "To break the enemy's front, in order to outflank the enemies present battle front." (Opposite the British Fourth Army at Amiens) Allied leaders had now realized that to continue an attack after resistance had hardened was a waste of lives and it was better to turn a line than to try and roll over it. Attacks were being undertaken in quick order to take advantage of the successful advances on the flanks and then broken off when that attack lost its initial impetus.

The British Third Army's 15-mile (24 km) front north of Albert progressed after stalling for a day against the main resistance line to which the enemy had withdrawn. Rawlinson’s Fourth British Army was able to battle its left flank forward between Albert and the Somme straightening the line between the advanced positions of the Third Army and the Amiens front which resulted in recapturing Albert at the same time. On August 26 the British First Army on the left of the Third Army was drawn into the battle extending it northward to beyond Arras. The Canadian Corps already being back in the vanguard of the First Army fought their way from Arras eastward 5 miles (8 km) astride the heavily defended Arras-Cambrai before reaching the outer defenses of the Hindenburg line, breaching them on the 28th and 29th. Bapaume fell on the 29th to the New Zealand Division of the Third Army and the Australians, still leading the advance of the Fourth Army, were again able to push forward at Amiens to take Peronne and Mont St. Quentin on August 31. Further south the French First and Third Armies had slowly fought forward while the Tenth Army, who had by now crossed the Ailette and was east of the Chemin des Dames, was now near to the Alberich position of the Hindenburg line. During the last week of August the pressure along a 70-mile (113 km) front against the enemy was heavy and unrelenting. From German accounts, "Each day was spent in bloody fighting against an ever and again on-storming enemy, and nights passed without sleep in retirements to new lines." Even to the north in Flanders the British Second and Fifth Armies during August and September were able to make progress taking prisoners and positions that were previously denied them.

On September 2 the Canadian Corps outflanking of the Hindenburg line, with the breaching of the Wotan Position, made it possible for the Third Army to advance and sent repercussions all along the Western Front. That same day OHL had no choice but to issue orders to six armies for withdrawal back into the Hindenburg line in the south, behind the Canal Du Nord on the Canadian-First Army's front and back to a line east of the Lys in the north, giving up without a fight the salient seized in the previous April.According to Ludendorff "We had to admit the necessity…to withdraw the entire front from the Scarpe to the Vesle."

In nearly four weeks of fighting since August 8 over 100,000 German prisoners were taken, 75,000 by the BEF and the rest by the French. Since "The Black Day of the German Army" the German High Command realized the war was lost and made attempts for a satisfactory end. The day after the battle Ludenforff told Colonel Mertz "We cannot win the war any more, but we must not lose it either." On August 11 he offered his resignation to the Kaiser, who refused it and replied, "I see that we must strike a balance. We have nearly reached the limit of our powers of resistance. The war must be ended." On August 13 at Spa, Hindenburg, Ludendorff, Chancellor and Foreign minister Hintz agreed that the war could not be ended militarily and on the following day the German Crown Council decided victory in the field was now most improbable. Austria and Hungary warned that they could only continue the war until December and Ludendorff recommended immediate peace negotiations, to which the Kaiser responded by instructing Hintz to seek the Queen of Holland's mediation. Prince Rupprecht warns Prince Max of Baden "Our military situation has deteriorated so rapidly that I no longer believe we can hold out over the winter; it is even possible that a catastrophe will come earlier." On September 10 Hindenburg urged peace moves to Emperor Charles of Austria and Germany appealed to Holland for mediation. On the 14th Austria sent a note to all belligerents and neutrals suggesting a meeting for peace talks on neutral soil and on September 15 Germany made a peace offer to Belgium. Both peace offers were rejected and on September 24 OHL informed the leaders in Berlin that armistice talks were inevitable.

September saw the Germans continuing to fight strong rear guard actions and launching numerous counter attacks on lost positions, with only a few succeeding and then only temporarily. Contested towns, villages, heights and trenches in the screening positions and outposts of the Hindenburg Line continued to fall to the Allies as well as thousands of prisoners, with the BEF alone taking 30,441 in the last week of September. Further small advances eastward would follow the Third Army victory at Ivincourt on September 12, the Fourth Armies at Epheny on the 18th and the French gain of Essigny Le Grand a day later. On the 24th a final assault by both the British and French on a four mile (6 km) front would come within two miles (3 km) of St. Quentin. With the outposts and preliminary defensive lines of the Seigfried and Alberich Positions eliminated the Germans were now completely back in the Hindenburg line. With the Wotan position of that line already breached and the Seigfried position in danger of being turned from the north the time had now come for an assault on the whole length of the line.

The Allied attack on the Hindenburg Line began on 26 September. 260,000 U.S. soldiers went "over the top". All initial objectives were captured; the U.S. 79th Infantry Division, which met stiff resistance at Montfaucon, took an extra day to capture its objective. The U.S. Army stalled because of supply problems because its inexperienced headquarters had to cope with large units and a difficult landscape. At the same time, French units broke through in Champagne and closed on the Belgian frontier. The most significant advance came from Commonwealth units, as they entered Belgium (liberation of Ghent). The German army had to shorten its front and use the Dutch frontier as an anchor to fight rear-guard actions.

By October, it was evident that Germany could no longer mount a successful defence. They were increasingly outnumbered, with few new recruits. Rations were cut. Ludendorff decided, on 1 October, that Germany had two ways out — total annihilation or an armistice. He recommended the latter at a summit of senior German officials. Allied pressure did not let up.

Meanwhile, news of Germany's impending military defeat spread throughout the German armed forces. The threat of mutiny was rife. Admiral Reinhard Scheer and Ludendorff decided to launch a last attempt to restore the "valour" of the German Navy. Knowing the government of Max von Baden would veto any such action, Ludendorff decided not to inform him. Nonetheless, word of the impending assault reached sailors at Kiel. Many rebelled and were arrested, refusing to be part of a naval offensive which they believed to be suicidal. Ludendorff took the blame—the Kaiser dismissed him on 26 October. The collapse of the Balkans meant that Germany was about to lose its main supplies of oil and food. The reserves had been used up, but U.S. troops kept arriving at the rate of 10,000 per day.

Having suffered over 6 million casualties, Germany moved toward peace. Prince Max von Baden took charge of a new government as Chancellor of Germany to negotiate with the Allies. Negotiations with President Wilson began immediately, in the vain hope that better terms would be offered than with the British and French. Instead Wilson demanded the abdication of the Kaiser. There was no resistance when the social democrat Philipp Scheidemann on 9 November declared Germany to be a republic. Imperial Germany was dead; a new Germany had been born: the Weimar Republic.