The United States remained neutral in the first three years of the First World War (1914 to 18) but did finally join the conflict at the end of 1917 following the threat of unrestricted submarine warfare on US merchant shipping and a secret telegram which suggested Germany and Mexico would become allies so that the latter could grab control of three southern states. The US government was also motivated to join WWI by a desire to uphold democratic principles and the freedom of small nations, and to control a post-war settlement in Europe and Eurasia.

The US contributed to the Allied war effort through material and financial aid and with all three armed services – land, air, and sea. Suffering some 325,000 casualties, US forces helped achieve Allied victories on the Western Front and in the North Sea and North Atlantic in the last year of a conflict that was responsible for over 28 million military casualties on all sides.

In the early 20th century, the United States was challenging Great Britain as the richest and most powerful country in the world, although the latter had a much larger navy. The First World War began in August 1914, following the assassination of Archduke Franz Ferdinand in Sarajevo and then various states declaring war on each other according to the pre-WWI alliance system. The Triple Entente of Britain, France, and Russia (aka the Allies) faced the Central Powers of Germany and Austria-Hungary. The US, like Italy, decided to remain neutral.

The United States president at the time was Woodrow Wilson (1856 to 1924), in office from 1913 to 1921. Wilson promised in February 1915 that Germany would be held "to a strict accountability" (Bruce, 405) for any US ships lost at sea due to aggressive acts. President Wilson became more active in 1916 and offered the United States' services as a mediator in Europe, but this was rejected by both sides. On the other side, the United States was not best pleased with the negative effect the Allied blockade of Germany was having on world trade and 'the freedom of the seas'. 1916 was also an election year, and Wilson, who won (albeit narrowly), based his campaign on his ability to keep the United States out of the war. Despite its neutrality, US banks did loan around $2 billion to the Allies to help them pay for war materials, a figure which rose to over $10 billion by the end of the war. Weapons, ammunition, and food were also sent across the Atlantic. US businesses did well out of the war: "Between 1915 and 1917 US exports doubled, with 65 per cent going to Great Britain." (Winter, 512).

Pressure for the United States to enter the war in military terms increased following the sinking of the British transatlantic liner RMS Lusitania by a German U-boat on 7 May 1915. The ship, which was on its way from New York to Liverpool, went down off the coast of Ireland, and of the 1,200 civilians that lost their lives, 128 victims were from the United States.

The United States almost broke off diplomatic relations with Germany over the disaster. The German government tried to justify the sinking by claiming – without producing any evidence – the Lusitania had been acting as an armed merchant cruiser. The liner had neither troops nor arms on board, although it was carrying small-calibre ammunition, which was allowed under the merchant shipping rules of the day. It was pointed out that a German embassy in the United States had previously warned civilians through newspaper advertisements of the dangers of travelling to Europe by sea at a time of war. Germany also defended the sinking by reminding everyone that it was fighting a blockade and had already declared the waters around the British Isles were a war zone and that any merchant vessel was fair game. The diplomatic furore did not, in the end, lead to the US joining the war, and after the Lusitania disaster, Germany limited its submarine attacks on merchant vessels.

The US entry into WWI came about largely thanks to the interception of a coded telegram, subsequently known as the Zimmermann telegram. The sender of the telegram was Arthur Zimmermann, German Foreign Secretary. The intended receiver was Count von Bernstorff, the German Ambassador in Washington. The telegram, sent in January 1917, mentioned Germany's decision to resume unrestricted submarine warfare. This was something the German government knew was risky and might lead to US military involvement in the war, but it was hoped that enough damage could be inflicted on Allied shipping to win the war before the US could mobilise its military. Even more damaging, the telegram included a proposal that Mexico and Germany become allies. Mexico was to declare war on the United States, and Germany would support its ambition of taking control of Texas, New Mexico, and Arizona. In a rather long telegram, Japan would be invited to join this German-Mexican anti-American alliance. The contents of the telegram were made public on 1 March. Zimmermann confirmed he had indeed sent such a telegram. The Mexican government denied everything. The American public was outraged. The sinking of four US merchant ships on 22 March only deepened the crisis. President Wilson appealed to Congress that a declaration of war should be made. Wilson declared that "the world must be made safe for democracy" (Bruce, 406).

The United States finally declared war on Germany on 6 April 1917 and then subsequently on Germany's allies. The financial and material aid given to the Allies was increased. The American Expeditionary Force (AEF) was dispatched to Europe to aid the Allies, but the United States remained an 'associate' rather than a full ally of Britain, France, Belgium, Italy (joined in 1915), and Russia, since Wilson was careful not to enter into any formal agreements with other states, agreements which might run against US interests once the war was won.

The US contribution to the war on land was limited by the size of its army, then only the 17th largest in the world. The AEF was small, inexperienced, and it took an age to actually get to Europe: By the end of November 1917, not even four full US divisions (130,000 men) were at or near the front. The AEF did have a capable leader in General John Pershing (1860 to 1948). Known as 'Black Jack' Pershing, the US commander was long on experience, including the much-publicised pursuit of Pancho Villa, one of the key leaders during the Mexican Revolution. Pershing troubled his superiors by immediately calling for 3 million US soldiers to be sent to Europe instead of the planned one million. In the end, 1.5 million US servicemen arrived to fight in France (Winter, 170). The numbers were still significant, certainly enough to seriously dent the morale of German frontline troops who now realised a powerful new enemy with seemingly inexhaustible resources had entered the arena.

The majority of US troops were selectively conscripted from young unmarried males without dependents. Known as 'doughboys' (a name of obscure origin), they required three months of further training in France before they could be sent into action. In addition, Pershing was adamant that only when sufficient numbers had been built up could the AEF effectively participate on the Western Front. Pershing and Wilson were also determined that the US force not be merely used as reinforcements to replenish depleted British and French divisions. There were important political reasons, too, for the hesitation; back home, the US intervention in the war had to be seen as successful, since many doubters over the US military's presence remained. Victories with a clear US involvement were what everybody wanted to see. There was the additional concern that joining US troops with Allied troops would muddy the political waters after the war, when the US might have objectives that differed from those of Britain and France.

Once it got going, the US army's presence did build rapidly through the first months of 1918. By March, there were 430,000 US troops in France, and by May, this had risen to 650,000. The US involvement was particularly welcomed by the Allies, since Russia had withdrawn from the conflict after the Bolshevik Revolution of November 1917. Still, the US high command's insistence that US troops remained a separate army angered Allied generals, who desperately needed more men at particular points of the front, especially since Germany was making one final big push for total victory.

US infantry divisions, without such means themselves, relied heavily on Allied ammunition, armour, and artillery, as well as air and logistical support. The doughboys were certainly popular with the locals and their comrades in arms, as here explained by J. Keegan: "Their personal popularity was everywhere noted. The Americans were light-hearted, cheerful, enthusiastic, dismissive of difficulties. 'We'll soon settle this,' was the doughboy attitude" (374).

US infantry troops finally saw battle in the German spring offensives of 1918. At first under the command of Allied generals after Pershing partially relented, the US troops then fought independently from July 1918 as the American First Army, mainly in the region east of Verdun. US successes included the capture of Cantigny from veteran German troops and victory at the Battle of Belleau Wood against a numerically superior enemy. Units that fought here included the most professional element of the AEF, the US Marine Corps. US troops were also involved in the latter stages of the Second Battle of the Marne in July 1918, which resulted in a German retreat, and the last Allied offensive on the Western Front, the Meuse-Argonne Offensive from September to November 1918, which persuaded the German generals that an armistice was now their only option. The United States Army suffered around 300,000 casualties in the war.

The United States Navy entered the war in April 1917 when a number of destroyers and submarines were sent to aid the Allies. These ships, the Fourth Squadron, saw service as part of a larger British fleet, specifically against the German U-boat threat. US ships, including five battleships, also played their part in tightening the Allied blockade of Germany and in laying an extensive stretch of mines in the North Sea. This belt of mines, which attempted to block the waters between Norway and Scotland, helped protect Britain's eastern coastline and was responsible for sinking five German submarines. As the war reached a close and Germany's tactics at sea became increasingly desperate, more US ships were sent to Europe, including more battleships, this time to protect Ireland. Queenstown in southern Ireland acted as the main base for the US fleet. By the end of the war, the US Navy had lost two submarines, two destroyers, and one armoured cruiser. In total, 389,000 tons of US shipping was sunk.

The United States Air Service was involved in WWI from September 1917. The air service was most famously led by Brigadier-General William Mitchell (1879 to 1936), but the service still came under the overall command of General Pershing. Flying its first combat missions in the spring of 1918, US pilots engaged enemy fighters and, that summer, conducted bombing raids on trenches along the Western Front. One battle of note which US pilots were involved in was the victorious Saint-Mihiel offensive in September 1918, the largest battle involving American troops since the American Civil War (1861 to 65). The most successful single US pilot was Captain Edward Rickenbacker (1890 to 1973), who, curiously enough, started the war as General Pershing's driver and ended it having shot down 22 enemy planes and four balloons.

Although the initial plans were much grander, the US still fielded 45 squadrons (800 pilots and 840 operational aircraft) in France by the closing stages of the war. As US aircraft prior to 1917 were all obsolete, the US Air Service mainly flew French-built aircraft such as the Spad biplanes. Mitchell's plan to launch heavy bombing raids against enemy targets inside Germany was prevented from becoming a reality by the armistice.

On the US Home Front, citizens were encouraged to help fund the war effort by buying War Savings Stamps from the US Treasury. Posters featuring heroines of the past, such as Joan of Arc (who defended France against English attacks in the 15th century), were aimed specifically at American women, as were recruitment drives for nurses to serve in US army field hospitals. People of all walks of life were encouraged to buy war bonds (Liberty Bonds) in campaigns which involved the top film stars of the day, such as Douglas Fairbanks and Mary Pickford. Cinema helped keep the war in people's minds with obvious propaganda films, but also with more comedic efforts, such as Charlie Chaplin's Shoulder Arms, which made fun of life in the US army's training camps.

The war brought social change, too, when so many men were removed from the workforce to enlist in the armed forces, as here explained by the social historian G. Brayson: "…white domestic servants often found factory work, and many black women moved from the land to service in their place." (Strachan, 153). Indeed, there was a movement of around half a million Black people from Southern states to the more industry-heavy Northern ones.

By January 1918, Wilson had already been thinking about a peace settlement, and he produced his famous list of requirements for peace: Woodrow Wilson's Fourteen Points. This list illustrated the United States' desire to be at the heart of any post-war peace settlement, ensuring its interests were best served in the new world order. The various points of Wilson's list called for freedom of the seas, free trade, disarmament, a redrawing of the map of Europe based on the principle of national self-determination, a commitment to open diplomacy with no more secret treaties, and the creation of an international assembly of nations.

The Allies went on to win the war, with Germany signing an armistice on 11 November 1918. The US armed services had suffered around 325,000 casualties, including around 115,600 deaths. This compares to around 3.2 million British and British Empire, 5.6 million French, and 6.6 million Russian military casualties. The United States government, despite Wilson's influence on the peace process and the Treaty of Versailles, and the president's personal campaign across the country, returned to an isolationist policy. The US did not ratify the Treaty of Versailles and did not join the new League of Nations, a fact which seriously weakened the possibility of a sustained peace in Europe and beyond.