During World War I (1914–1918), belligerents from both the Allied Powers and Central Powers violated international criminal law, committing numerous war crimes. This includes the use of indiscriminate violence and massacres against civilians, torture, sexual violence, forced deportation and population transfer, death marches, the use of chemical weapons and the intentional targeting of Red Cross personnel and medical facilities.

The governments of all major combatants had previously signed the Hague Conventions of 1899 and 1907, which these atrocities intentionally violated. Even so, both the decisions to commit, and to refuse to court-martial, the perpetrators of World War I crimes was motivated by what American Civil War historian Thomas Lowry has termed "the European tradition … that to victors belong the spoils - the losers could expect pillage and plunder",Thomas P. Lowry, MD (1997), Tarnished Eagles: The Courts-Martial of Fifty Union Colonels and Lieutenant Colonels, Stackpoole Books. p. 161. and that enemy civilians are "grist for the mills of more hardheaded conquerors such as Genghis Khan, Tamerlane, and Ivan the Terrible."Thomas P. Lowry, MD (1997), Tarnished Eagles: The Courts-Martial of Fifty Union Colonels and Lieutenant Colonels, Stackpoole Books. p. 163.

Austro-Hungarian war crimes

Austro-Hungarian invasion and occupation of Serbia

Collective punishment and massacres of Serbs by Austria-Hungary During the first invasion of Serbia in 1914, Austro-Hungarian forces occupied parts of the country for 13 days. Their war aims were not only to eliminate Serbia as a threat, but also to punish the whole nation for the assassination of Archduke Franz Ferdinand. The invasion and military occupation accordingly turned into a war of annihilation, accompanied by massacres of civilians and the taking and summary execution of hostages. Austro-Hungarian troops committed a number of war crimes against the Serbs, especially in the area of Mačva, where according to historian Geoffrey Wawro, the Austro-Hungarian army subjected the civilian population to a wave of atrocities. During this short occupation, between 3,500 and 4,000 Serb civilians were killed in executions and acts of random violence by marauding troops.Mass killings took place in numerous towns in northern Serbia. On 17 August 1914, in Šabac, 120 residents—mostly women, children and old men—were shot and buried in a churchyard by Austro-Hungarian troops on the orders of Feldmarschall-Leutnant Kasimir von Lütgendorf. The remaining residents were beaten to death, hanged, stabbed, mutilated or burned alive. A pit was later discovered in the village of Lešnica containing 109 dead peasants who were "bound together with a rope and encircled by wire"; they had been shot and immediately buried, even with some still alive. A claim from a local spy that "traitors" were hiding in a certain house was enough to sentence the whole family to death by hanging. Serbian Orthodox priests were often summarily hanged, under the accusation of spreading the spirit of insurrection among local people. Victims were usually hanged on the main squares of villages and towns, in full view of the general population. The lifeless bodies were left to hang by the noose for several days as an act of intimidation.

Austria's propaganda machinery spread anti-Serb sentiment with the slogan "Serbien muss sterbien" (Serbia must die). During the war, Austro-Hungarian officers in Serbia ordered troops to "exterminate and burn everything that is Serbian", and hangings and mass shootings were everyday occurrences. Austrian historian Anton Holzer wrote that the Austro-Hungarian army carried out "countless and systematic massacres…against the Serbian population. The soldiers invaded villages and rounded up unarmed men, women and children. They were either shot dead, bayoneted to death or hanged. The victims were locked into barns and burned alive. Women were sent up to the front lines and mass-raped. The inhabitants of whole villages were taken as hostages, humiliated and tortured." Multiple source state that 30,000 Serbs, mostly civilians, were executed by Austro-Hungarian forces by the end of 1914.

Forced displacement and starvation of Serbs After being occupied completely in early 1916, both Austria-Hungary and Bulgaria announced that Serbia had ceased to exist as a political entity, and that its inhabitants could therefore not invoke the international rules of war dictating the treatment of civilians as defined by the Geneva Conventions and the Hague Conventions.

The Military General Governorate of Serbia (MGG/S), as well as the High Command in Vienna, considered sending civilian prisoners to internment camps. During the occupation, between 150,000 and 200,000 men, women and children were deported to various camps in Austria-Hungary; it has been estimated they represented slightly more than 10 per cent of the Serb population. Since Serbia did not have its own Red Cross, Serbian prisoners did not have access to the aid the Red Cross provided to other Allied prisoners. Moreover, Serbian prisoners were not considered "enemy aliens" but "internal enemies" by Austria-Hungary's Ministry of War. By defining them as "terrorists" or "insurgents", the Austro-Hungarian authorities were not obliged to disclose the number of captives they held, and which camps they were being held in, to Red Cross societies.

Serbs also suffered from famine; General Franz Conrad von Hötzendorf gave orders for Serbia's resources be "squeezed dry" regardless of the consequences for the population. Looting by occupying soldiers, combined with the food exporting policies of Austria and Germany, caused mass starvation, leading to the deaths of 8,000 Serbians during the winter of 1916. According to a Red Cross report dated 1 February 1918, by the end of 1917, there were 206,500 prisoners of war and internees from Serbia in Austro-Hungarian and German camps. According to the historian Alan Kramer, the Serbians in Austro-Hungarian captivity received the worst treatment of all the prisoners, and at least 30,000–40,000 had died of starvation by January 1918.

Thalerhof Internment Camp

Austrian authorities rounded up civilians from the province of Galicia and sent them to internment camps, on charges of being part of the Galician Russophilia movement.

Doboj Internment Camp 45,971 Bosnian Serbs were sent to an internment camp near the city of Doboj, Bosnia between 27 December 1915 and 5 July 1917.

British and Commonwealth war crimes

Royal Navy war crimes

On 19 August 1915, the German submarine U-27 was sunk by the British Q-ship . All German survivors were killed by Baralongs crew on the orders of Lieutenant-Commander Godfrey Herbert, the captain of the ship. The shooting was reported to the media by American citizens who were on board the Nicosian, a British freighter loaded with war supplies, which was stopped by U-27 just minutes before the incident.Halpern, Paul G. (1994). A Naval History of World War I. Routledge, p. 301;

On 24 September, Baralong destroyed U-41, which was in the process of sinking the cargo ship Urbino. According to a survivor from the submarine, Baralong continued to fly the US flag after firing on U-41 and then rammed the lifeboat carrying the German survivors, sinking it.Hadley, Michael L. (1995). Count Not the Dead: The Popular Image of the German Submarine. McGill-Queen's Press – MQUP, p. 36; .

Blockade of Germany

After the war, the German government claimed that approximately 763,000 German civilians died from starvation and disease during the war because of the Allied blockade. An academic study done in 1928 put the death toll at 424,000.Grebler, Leo (1940). The Cost of the World War to Germany and Austria–Hungary. Yale University Press. p. 78 Germany protested that the Allies had used starvation as a weapon of war. Sally Marks argued that the German accounts of a hunger blockade are a "myth", as Germany did not face the starvation level of Belgium and the regions of Poland and northern France that it occupied. Nevertheless, the eventual form of the blockade by mid-1915, being a distant blockade that made no distinction between civilian and military goods, was opposed to both the letter and spirit of the agreed but unratified 1909 London Declaration. This drew initial protest from neutral powers, though over the course of the war they would eventually cooperate with the Allies.

The armistice in November 1918 specified that the blockade would continue until a formal peace agreement. Thus, foodstuffs imports into Germany continued to be controlled by the Allies until German authorities signed the Treaty of Versailles in June 1919. In March 1919, Winston Churchill urged the House of Commons to come to a swift agreement as "Germany is very near starvation." From January 1919 to March 1919, Germany refused to agree to Allied demands that it surrender its merchant ships to Allied ports to transport food supplies. Some Germans considered the armistice to be a temporary cessation of the war and knew, if fighting broke out again, their ships would be seized. Over the winter of 1919, the situation became desperate and Germany finally agreed to surrender its fleet in March. The Allies then allowed for the import of 270,000 tons of foodstuffs.

Both German and non-German observers have argued that the period after the armistice were the most devastating months of the blockade for German civilians, though disagreement persists as to the extent and who is truly at fault. According to Max Rubner, 100,000 German civilians died due to the continued blockade after the armistice. In the UK, Labour Party member and anti-war activist Robert Smillie issued a statement in June 1919 condemning continuation of the blockade, claiming 100,000 German civilians had died as a result.Common Sense (London) 5 July 1919.

Internment of German-Australians Before a pervasive climate among the Australian guards of excessive cruelty and unnecessary brutality led to an international diplomatic incident, the dismissal with disgrace of their commanding officer, Captain G.E. Hawkes, from the Australian Army, and its permanent closure in 1916, the Torrens Island Concentration Camp in South Australia was used to hold both Central Powers POWs and civilian internees.Torrens Island Revelations, The Mail, 17 May 1919. Trove, National Library of Australia. Retrieved 6 January 2012.Chronicle, 30 November 1918.National Archives of Australia Series MP367/1 Item 567/3/2202 Captain G.E. Hawkes Court of Enquiry 1915–1919 (Some images online at http://naa12.naa.gov.au/scripts/Imagine.asp?B=5809579)

Internment of Ukrainian Canadians

Surafend massacre

Bulgarian war crimes

Bulgarian massacres of Serbs

Bulgarian Tsar Ferdinand declared on the eve of war: "the purpose of my life is the destruction of Serbia". Many Bulgarian troops were side-lined from front line duty to take part in the occupation of Serbia, past animosities led to brutality, the local population was left a choice between Bulgarisation or being subject to violence, large scale deportations and the treatment of the residents of the occupation zones came close to genocidal actions.

The , a report covering alleged atrocities committed in Serbia, published after the war, stated that ‘anyone unwilling to submit him or herself to the occupiers and become Bulgarian was tortured, raped, interned, and killed in particularly gruesome manners, some of which recorded photographically'. Bulgarian units that occupied Serbian territories showed extreme brutality, systematically expelling the non-Bulgarian population in the regions they occupied, they arrested the population and set the rebel villages on fire.