World War I
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). In Germany, civilian deaths were 474,000 higher than in peacetime, due in large part to food shortages and malnutrition that had weakened disease resistance. These excess deaths are estimated as 271,000 in 1918, plus another 71,000 in the first half of 1919 when the blockade was still in effect. Starvation caused by famine killed approximately 100,000 people in Lebanon.

Diseases flourished in the chaotic wartime conditions. In 1914 alone, louse-borne epidemic typhus killed 200,000 in Serbia. Starting in early 1918, a major influenza epidemic known as Spanish flu spread across the world, accelerated by the movement of large numbers of soldiers, often crammed together in camps and transport ships with poor sanitation. The Spanish flu killed at least 17 to 25 million people, including an estimated 2.64 million Europeans and as many as 675,000 Americans. Between 1915 and 1926, an epidemic of encephalitis lethargica affected nearly five million people worldwide.
Eight million equines mostly horses, donkeys and mules died, three-quarters of them from the extreme conditions they worked in.

War crimes: 

Chemical weapons in warfare: The German army was the first to successfully deploy chemical weapons during the Second Battle of Ypres (April–May 1915), after German scientists under the direction of Fritz Haber at the Kaiser Wilhelm Institute developed a method to weaponize chlorine. The use of chemical weapons had been sanctioned by the German High Command to force Allied soldiers out of their entrenched positions, complementing rather than supplanting more lethal conventional weapons. Chemical weapons were deployed by all major belligerents throughout the war, inflicting approximately 1.3 million casualties, of which about 90,000 were fatal. The use of chemical weapons in warfare was a direct violation of the 1899 Hague Declaration Concerning Asphyxiating Gases and the 1907 Hague Convention on Land Warfare, which prohibited their use.

Genocides by the Ottoman Empire: The ethnic cleansing of the Ottoman Empire's Armenian population, including mass deportations and executions, during the final years of the Ottoman Empire is considered genocide. The Ottomans carried out organised and systematic massacres of the Armenian population at the beginning of the war and manipulated acts of Armenian resistance by portraying them as rebellions to justify further extermination. In early 1915, several Armenians volunteered to join the Russian forces and the Ottoman government used this as a pretext to issue the Tehcir Law (Law on Deportation), which authorised the deportation of Armenians from the Empire's eastern provinces to Syria between 1915 and 1918. The Armenians were intentionally marched to death and a number were attacked by Ottoman brigands. While the exact number of deaths is unknown, the International Association of Genocide Scholars estimates around 1.5 million. The government of Turkey continues to deny the genocide to the present day, arguing that those who died were victims of inter-ethnic fighting, famine, or disease during World War I; these claims are rejected by most historians.
Other ethnic groups were similarly attacked by the Ottoman Empire during this period, including Assyrians and Greeks, and some scholars consider those events to be part of the same policy of extermination. At least 250,000 Assyrian Christians, about half of the population, and 350,000–750,000 Anatolian and Pontic Greeks were killed between 1915 and 1922.

Prisoners of war: About 8 million soldiers surrendered and were held in POW camps during the war. All nations pledged to follow the Hague Conventions on fair treatment of prisoners of war, and the survival rate for POWs was generally much higher than that of combatants at the front.
Around 25–31% of Russian losses (as a proportion of those captured, wounded, or killed) were to prisoner status; for Austria-Hungary 32%; for Italy 26%; for France 12%; for Germany 9%; for Britain 7%. Prisoners from the Allied armies totalled about 1.4 million (not including Russia, which lost 2.5–3.5 million soldiers as prisoners). From the Central Powers, about 3.3 million soldiers became prisoners; most of them surrendered to Russians.

Soldiers' experiences: Allied personnel was around 42,928,000, while Central personnel was near 25,248,000. British soldiers of the war were initially volunteers but were increasingly conscripted. Surviving veterans returning home often found they could discuss their experiences only among themselves, so formed "veterans' associations" or "Legions".

Conscription: Conscription was common in most European countries. However, it was controversial in English-speaking countries, It was especially unpopular among minority ethnicities—especially the Irish Catholics in Ireland, Australia, and the French Catholics in Canada.
In the US, conscription began in 1917 and was generally well-received, with a few pockets of opposition in isolated rural areas. The administration decided to rely primarily on conscription, rather than voluntary enlistment, to raise military manpower after only 73,000 volunteers enlisted out of the initial 1 million target in the first six weeks of war.

Military attachés and war correspondents: Military and civilian observers from every major power closely followed the course of the war. Many were able to report on events from a perspective somewhat akin to modern "embedded" positions within the opposing land and naval forces.

Economic effects: Macro- and micro-economic consequences devolved from the war. Families were altered by the departure of many men. With the death or absence of the primary wage earner, women were forced into the workforce in unprecedented numbers. At the same time, the industry needed to replace the lost labourers sent to war. This aided the struggle for voting rights for women.
In all nations, the government's share of GDP increased, surpassing 50% in both Germany and France and nearly reaching that level in Britain. To pay for purchases in the US, Britain cashed in its extensive investments in American railroads and then began borrowing heavily from Wall Street. President Wilson was on the verge of cutting off the loans in late 1916 but allowed a great increase in US government lending to the Allies. After 1919, the US demanded repayment of these loans. The repayments were, in part, funded by German reparations that, in turn, were supported by American loans to Germany. This circular system collapsed in 1931 and some loans were never repaid. Britain still owed the United States $4.4 billion of World War I debt in 1934; the last installment was finally paid in 2015.
Britain turned to her colonies for help in obtaining essential war materials whose supply from traditional sources had become difficult. Geologists such as Albert Kitson were called on to find new resources of precious minerals in the African colonies. Kitson discovered important new deposits of manganese, used in munitions production, in the Gold Coast.
Article 231 of the Treaty of Versailles (the so-called "war guilt" clause) stated Germany accepted 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 the aggression of Germany and her allies." It was worded as such to lay a legal basis for reparations, and a similar clause was inserted in the treaties with Austria and Hungary. However, neither of them interpreted it as an admission of war guilt. In 1921, the total reparation sum was placed at 132 billion gold marks. However, "Allied experts knew that Germany could not pay" this sum. The total sum was divided into three categories, with the third being "deliberately designed to be chimerical" and its "primary function was to mislead public opinion ... into believing the 'total sum was being maintained.'" Thus, 50 billion gold marks (12.5 billion dollars) "represented the actual Allied assessment of German capacity to pay" and "therefore ... represented the total German reparations" figure that had to be paid.
This figure could be paid in cash or in-kind (coal, timber, chemical dyes, etc.). Some of the territory lost—via the Treaty of Versailles—was credited towards the reparation figure as were other acts such as helping to restore the Library of Louvain. By 1929, the Great Depression caused political chaos throughout the world. In 1932 the payment of reparations was suspended by the international community, by which point Germany had paid only the equivalent of 20.598 billion gold marks. With the rise of Adolf Hitler, all bonds and loans that had been issued and taken out during the 1920s and early 1930s were cancelled.