Thomas Woodrow Wilson (December 28, 1856 February 3, 1924) was an American politician and academic who served as the 28th president of the United States from 1913 to 1921. A member of the Democratic Party, Wilson served as the president of Princeton University and as the governor of New Jersey before winning the 1912 presidential election. As President, Wilson changed the nation's economic policies and led the United States into World War I in 1917. He was the leading architect of the League of Nations, and his progressive stance on foreign policy came to be known as Wilsonianism.

Wilson grew up in the American South, mainly in Augusta, Georgia, during the Civil War and Reconstruction. After earning a Ph.D. in political science from Johns Hopkins University, Wilson taught at various colleges before becoming the president of Princeton University and a spokesman for progressivism in higher education. As governor of New Jersey from 1911 to 1913, Wilson broke with party bosses and won the passage of several progressive reforms. To win the presidential nomination he mobilized progressives and Southerners to his cause at the 1912 Democratic National Convention. Wilson defeated incumbent Republican William Howard Taft and third-party nominee Theodore Roosevelt to easily win the 1912 United States presidential election, becoming the first Southerner to do so since 1848.

During his first year as president, Wilson authorized the widespread imposition of segregation inside the federal bureaucracy. His first term was largely devoted to pursuing passage of his progressive New Freedom domestic agenda. His first major priority was the Revenue Act of 1913, which lowered tariffs and began the modern income tax. Wilson also negotiated the passage of the Federal Reserve Act, which created the Federal Reserve System. Two major laws, the Federal Trade Commission Act and the Clayton Antitrust Act, were enacted to promote business competition and combat extreme corporate power.

At the outbreak of World War I in 1914, the U.S. declared neutrality as Wilson tried to negotiate a peace between the Allied and Central Powers. He narrowly won re-election in the 1916 United States presidential election, boasting how he kept the nation out of wars in Europe and Mexico. In April 1917, Wilson asked Congress for a declaration of war against Germany in response to its policy of unrestricted submarine warfare that sank American merchant ships. Wilson nominally presided over war-time mobilization and left military matters to the generals. He instead concentrated on diplomacy, issuing the Fourteen Points that the Allies and Germany accepted as a basis for post-war peace. He wanted the off-year elections of 1918 to be a referendum endorsing his policies, but instead the Republicans took control of Congress. After the Allied victory in November 1918, Wilson went to Paris where he and the British and French leaders dominated the Paris Peace Conference. Wilson successfully advocated for the establishment of a multinational organization, the League of Nations. It was incorporated into the Treaty of Versailles that he signed. Wilson had refused to bring any leading Republican into the Paris talks, and back home he rejected a Republican compromise that would have allowed the Senate to ratify the Versailles Treaty and join the League.

Wilson had intended to seek a third term in office but suffered a severe stroke in October 1919 that left him incapacitated. His wife and his doctor controlled Wilson, and no significant decisions were made. Meanwhile, his policies alienated German and Irish Democrats and the Republicans won a landslide in the 1920 presidential election. Scholars have generally ranked Wilson in the upper tier of U.S presidents, although he has been criticized for supporting racial segregation. His liberalism nevertheless lives on as a major factor in American foreign policy, and his vision of ethnic self-determination resonated globally.

World War I, often abbreviated as WWI or WW1, also known as the First World War and contemporaneously known as the Great War and by other names, was an international conflict that began on 28 July 1914 and ended on 11 November 1918. It involved much of Europe, as well as Russia, the United States and Turkey, and was also fought in the Middle East, Africa and parts of Asia. One of the deadliest conflicts in history, an estimated 9 million were killed in combat, while over 5 million civilians died from occupation, bombardment, hunger or disease. The genocides perpetrated by the Ottomans and the 1918 Spanish flu pandemic spread by the movement of combatants during the war caused many millions of additional deaths worldwide.In 1914, the Great Powers were divided into two opposing alliances: the Triple Entente, consisting of France, Russia, and Britain, and the Triple Alliance, made up of Germany, Austria-Hungary, and Italy. Tensions in the Balkans came to a head on 28 June 1914 following the assassination of Archduke Franz Ferdinand, the Austro-Hungarian heir, by Gavrilo Princip, a Bosnian Serb. Austria-Hungary blamed Serbia and the interlocking alliances involved the Powers in a series of diplomatic exchanges known as the July Crisis. On 28 July, Austria-Hungary declared war on Serbia; Russia came to Serbia's defence and by 4 August, the conflict had expanded to include Germany, France and Britain, along with their respective colonial empires. In November, the Ottoman Empire, Germany and Austria formed the Central Powers, while in April 1915, Italy joined Britain, France, Russia and Serbia as the Allied Powers.

Facing a war on two fronts, German strategy in 1914 was to defeat France, then shift its forces to the East and knock out Russia, commonly known as the Schlieffen Plan. This failed when their advance into France was halted at the Marne; by the end of 1914, the two sides faced each other along the Western Front, a continuous series of trench lines stretching from the Channel to Switzerland that changed little until 1917. By contrast, the Eastern Front was far more fluid, with Austria-Hungary and Russia gaining, then losing large swathes of territory. Other significant theatres included the Middle East, the Alpine Front and the Balkans, bringing Bulgaria, Romania and Greece into the war.

Shortages caused by the Allied naval blockade led Germany to initiate unrestricted submarine warfare in early 1917, bringing the previously neutral United States into the war on 6 April 1917. In Russia, the Bolsheviks seized power in the 1917 October Revolution and made peace in the March 1918 Treaty of Brest-Litovsk, freeing up large numbers of German troops. By transferring these to the Western Front, the German General Staff hoped to win a decisive victory before American reinforcements could impact the war, and launched the March 1918 German spring offensive. Despite initial success, it was soon halted by heavy casualties and ferocious defence; in August, the Allies launched the Hundred Days Offensive and although the German army continued to fight hard, it could no longer halt their advance.Towards the end of 1918, the Central Powers began to collapse; Bulgaria signed an Armistice on 29 September, followed by the Ottomans on 31 October, then Austria-Hungary on 3 November. Isolated, facing revolution at home and an army on the verge of mutiny, Kaiser Wilhelm abdicated on 9 November and the new German government signed the Armistice of 11 November 1918, bringing the fighting to a close. The 1919 Paris Peace Conference imposed various settlements on the defeated powers, the best known being the Treaty of Versailles. The dissolution of the Russian, German, Ottoman and Austro-Hungarian empires led to numerous uprisings and the creation of independent states, including Poland, Czechoslovakia and Yugoslavia. For reasons that are still debated, failure to manage the instability that resulted from this upheaval during the interwar period ended with the outbreak of World War II in 1939.