World War I
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. On 29 October, the imperial authorities asked Italy for an armistice, but the Italians continued advancing, reaching Trento, Udine, and Trieste. On 3 November, Austria-Hungary sent a flag of truce and accepted the Armistice of Villa Giusti, arranged with the Allied Authorities in Paris. Austria and Hungary signed separate armistices following the overthrow of the Habsburg monarchy. In the following days, the Italian Army occupied Innsbruck and all Tyrol, with over 20,000 soldiers.
On 30 October, the Ottoman Empire capitulated, and signed the Armistice of Mudros.

German government surrenders: With the military faltering and with widespread loss of confidence in the Kaiser leading to his abdication and fleeing of the country, Germany moved towards surrender. Prince Maximilian of Baden took charge on October 3 as Chancellor of Germany. Negotiations with President Wilson began immediately, in the hope that he would offer better terms than the British and French. Wilson demanded a constitutional monarchy and parliamentary control over the German military.
The German Revolution of 1918–1919 began at the end of October 1918. Units of the German Navy refused to set sail for a last, large-scale operation in a war they believed to be as good as lost. The sailors' revolt, which then ensued in the naval ports of Wilhelmshaven and Kiel, spread across the whole country within days and led to the proclamation of a republic on 9 November 1918, shortly thereafter to the abdication of Kaiser Wilhelm II, and German surrender.

Aftermath: In the aftermath of the war, the German, Austro-Hungarian, Ottoman, and Russian empires disappeared. Numerous nations regained their former independence, and new ones were created. Four dynasties fell as a result of the war: the Romanovs, the Hohenzollerns, the Habsburgs, and the Ottomans. Belgium and Serbia were badly damaged, as was France, with 1.4 million soldiers dead, not counting other casualties. Germany and Russia were similarly affected.

Formal end of the war: A formal state of war between the two sides persisted for another seven months, until the signing of the Treaty of Versailles with Germany on 28 June 1919. The US Senate did not ratify the treaty despite public support for it, and did not formally end its involvement in the war until the Knox–Porter Resolution was signed on 2 July 1921 by President Warren G. Harding. For the British Empire, the state of war ceased under the provisions of the Termination of the Present War (Definition) Act 1918 concerning:

Germany on 10 January 1920.
Austria on 16 July 1920.
Bulgaria on 9 August 1920.
Hungary on 26 July 1921.
Turkey on 6 August 1924.

Some war memorials date the end of the war as being when the Versailles Treaty was signed in 1919, which was when many of the troops serving abroad finally returned home; by contrast, most commemorations of the war's end concentrate on the armistice of 11 November 1918.

Peace treaties and national boundaries: The Paris Peace Conference imposed a series of peace treaties on the Central Powers officially ending the war. The 1919 Treaty of Versailles dealt with Germany and, building on Wilson's 14th point, established the League of Nations on 28 June 1919.
The Central Powers had to acknowledge responsibility for "all the loss and damage to which the Allied and Associated Governments and their nationals have been subjected as a consequence of the war imposed upon them by" their aggression. In the Treaty of Versailles, this statement was Article 231. This article became known as the "War Guilt Clause", as the majority of Germans felt humiliated and resentful. The Germans felt they had been unjustly dealt with by what they called the "diktat of Versailles". German historian Hagen Schulze said the Treaty placed Germany "under legal sanctions, deprived of military power, economically ruined, and politically humiliated." Belgian historian Laurence Van Ypersele emphasises the central role played by memory of the war and the Versailles Treaty in German politics in the 1920s and 1930s:

Active denial of war guilt in Germany and German resentment at both reparations and continued Allied occupation of the Rhineland made widespread revision of the meaning and memory of the war problematic. The legend of the "stab in the back" and the wish to revise the "Versailles diktat", and the belief in an international threat aimed at the elimination of the German nation persisted at the heart of German politics. Even a man of peace such as [Gustav] Stresemann publicly rejected German guilt. As for the Nazis, they waved the banners of domestic treason and international conspiracy in an attempt to galvanise the German nation into a spirit of revenge. Like a Fascist Italy, Nazi Germany sought to redirect the memory of the war to the benefit of its policies.
Meanwhile, new nations liberated from German rule viewed the treaty as a recognition of wrongs committed against small nations by much larger aggressive neighbours.Austria-Hungary was partitioned into several successor states, largely but not entirely along ethnic lines. Apart from Austria and Hungary, Czechoslovakia, Italy, Poland, Romania and Yugoslavia received territories from the Dual Monarchy (the formerly separate and autonomous Kingdom of Croatia-Slavonia was incorporated into Yugoslavia). The details were contained in the treaties of Saint-Germain-en-Laye and Trianon. As a result, Hungary lost 64% of its total population, decreasing from 20.9 million to 7.6 million, and losing 31% (3.3 out of 10.7 million) of its ethnic Hungarians. According to the 1910 census, speakers of the Hungarian language included approximately 54% of the entire population of the Kingdom of Hungary. Within the country, numerous ethnic minorities were present: 16.1% Romanians, 10.5% Slovaks, 10.4% Germans, 2.5% Ruthenians, 2.5% Serbs and 8% others. Between 1920 and 1924, 354,000 Hungarians fled former Hungarian territories attached to Romania, Czechoslovakia, and Yugoslavia.
The Russian Empire lost much of its western frontier as the newly independent nations of Estonia, Finland, Latvia, Lithuania, and Poland were carved from it. Romania took control of Bessarabia in April 1918.

National identities: After 123 years, Poland re-emerged as an independent country. The Kingdom of Serbia and its dynasty, as a "minor Entente nation" and the country with the most casualties per capita, became the backbone of a new multinational state, the Kingdom of Serbs, Croats and Slovenes, later renamed Yugoslavia. Czechoslovakia, combining the Kingdom of Bohemia with parts of the Kingdom of Hungary, became a new nation. Romania would unite all Romanian-speaking people under a single state, leading to Greater Romania.
In Australia and New Zealand, the Battle of Gallipoli became known as those nations' "Baptism of Fire". It was the first major war in which the newly established countries fought, and it was one of the first times that Australian troops fought as Australians, not just subjects of the British Crown, and independent national identities for these nations took hold. Anzac Day, commemorating the Australian and New Zealand Army Corps (ANZAC), celebrates this defining moment.
In the aftermath of World War I, Greece fought against Turkish nationalists led by Mustafa Kemal, a war that eventually resulted in a massive population exchange between the two countries under the Treaty of Lausanne. According to various sources, several hundred thousand Greeks died during this period, which was tied in with the Greek genocide.

Casualties: Of the 60 million European military personnel who were mobilised from 1914 to 1918, an estimated 8 million were killed, 7 million were permanently disabled, and 15 million were seriously injured. Germany lost 15.1% of its active male population, Austria-Hungary lost 17.1%, and France lost 10.5%. France mobilised 7.8 million men, of which 1.4 million died and 3.2 million were injured. Approximately 15,000 deployed men sustained gruesome facial injuries, causing social stigma and marginalisation; they were called the gueules cassées (broken faces).