List of war crimes
1899–1902 Second Boer War: The term "concentration camp" was used to describe camps operated by the British Empire in South Africa during the Second Boer War in the years 1900–1902. As Boer farms were destroyed by the British under their "scorched earth" policy, many tens of thousands of women and children were forcibly moved into the concentration camps. Over 26,000 Boer women and children were to perish in these concentration camps.
Six officers from the Bushveldt Carbineers were court-martialed for massacring POWs and civilians. Lieutenants Harry Morant, Peter Handcock, and George Witton were each found guilty of murder and sentenced to death. Morant and Handcock were executed, while Witton was reprieved and served a short prison sentence. Two of the other defendants, Major Robert Lenehan and Lieutenant Henry Picton, were found guilty of lesser charges. They were dismissed from the military and deported from South Africa after being found guilty of neglecting one's duty and manslaughter, respectively. The last defendant, Captain Alfred Taylor, was acquitted.

1899–1902 Philippine–American War: Reported American war crimes and atrocities during the Philippine–American War included the summary execution of civilians and prisoners, burning of villages, and torture. 298,000 Filipinos were also moved to concentration camps, where thousands died.
In November 1901, the Manila correspondent of the Philadelphia Ledger wrote: "The present war is no bloodless, opera bouffe engagement; our men have been relentless, have killed to exterminate men, women, children, prisoners and captives, active insurgents and suspected people from lads of ten up, the idea prevailing that the Filipino as such was little better than a dog".
In response to the Balangiga massacre, which wiped out a U.S. company garrisoning Samar town, U.S. Brigadier General Jacob H. Smith launched a retaliatory march across Samar with the instructions: "I want no prisoners. I wish you to kill and burn, the more you kill and burn the better it will please me. I want all persons killed who are capable of bearing arms in actual hostilities against the United States".

1904–1908: Herero Wars: In August, German General Lothar von Trotha defeated the Ovaherero in the Battle of Waterberg and drove them into the desert of Omaheke, where most of them died of dehydration. In October, the Nama people also rebelled against the Germans, only to suffer a similar fate. Between 24,000 and 100,000 Hereros, 10,000 Nama and an unknown number of San died in the parallel Herero and Namaqua genocide. Once defeated, thousands of Hereros and Namas were also imprisoned in concentration camps, where the majority died of diseases, abuse, and exhaustion. German soldiers also regularly engaged in gang rapes before killing the women or leaving them in the desert to die; a number of Herero women were also forced into involuntary prostitution.: 31

1912-1913: Balkan Wars: The Balkan Wars were marked by ethnic cleansing with all parties being responsible for grave atrocities against civilians and helped inspire later atrocities including war crimes during the 1990s Yugoslav Wars.
Massacres of Albanians in the Balkan Wars were perpetrated on several occasions by Serbian and Montenegrin armies and paramilitaries. According to contemporary accounts, between 10,000 and 25,000 Albanians were killed or died because of hunger and cold during that period; many of the victims were children, women and the elderly. In addition to the massacres, some civilians had their tongues, lips, ears and noses severed. Philip J. Cohen also cited Durham as saying that Serbian soldiers helped bury people alive in Kosovo. American relief commissioner Willar Howard said in a 1914 Daily Mirror interview that General Carlos Popovitch would shout, "Don't run away, we are brothers and friends. We don't mean to do any harm." Peasants who trusted Popovitch were shot or burned to death, and elderly women unable to leave their homes were also burned. Yugoslavia from a Historical Perspective, a 2017 study published in Belgrade by the Helsinki Committee for Human Rights in Serbia, said that villages were burned to ashes and Albanian Muslims forced to flee when Serbo-Montenegrin forces invaded Kosovo in 1912. Some chronicles cited decapitation as well as mutilation.
Serbian army also brutally suppressed the Tikveš uprising and terrorized the Bulgarian population in the rebelling regions. According to some sources 363 civilian Bulgarians were killed in Kavadarci, 230 - in Negotino and 40 - in Vatasha.

1914–1918: World War I: World War I was the first major international conflict to take place following the codification of war crimes at the Hague Convention of 1907, including derived war crimes, such as the use of poisons as weapons, as well as crimes against humanity, and derivative crimes against humanity, such as torture, and genocide. Before, the Second Boer War took place after the Hague Convention of 1899. The Second Boer War (1899 until 1902) is known for the first concentration camps (1900 until 1902) for civilians in the 20th century.

1915–1920: First and Second Caco War: During the First (1915) and Second (1918–1920) Caco Wars waged during the United States occupation of Haiti (1915–1934), human rights abuses were committed against the native Haitians population. Overall, American troops and the Haitian gendarmerie killed several thousands of Haitian civilians during the rebellions between 1915 and 1920, though the exact death toll is unknown.
Mass killings of civilians were allegedly committed by United States Marines and their subordinates in the Haitian gendarmerie. According to Haitian historian Roger Gaillard, such killings involved rape, lynchings, summary executions, burning villages and deaths by burning. Internal documents of the United States Army justified the killing of women and children, describing them as "auxiliaries" of rebels. A private memorandum of the Secretary of the Navy criticized "indiscriminate killings against natives". American officers who were responsible for acts of violence were given Creole names such as "Linx" for Commandant Freeman Lang and "Ouiliyanm" for Lieutenant Lee Williams. According to American journalist H. J. Seligman, Marines would practice "bumping off Gooks", describing the shooting of civilians in a manner which was similar to killing for sport.
During the Second Caco War of 1918–1919, many Caco prisoners were summarily executed by Marines and the gendarmerie on orders from their superiors. On June 4, 1916, Marines executed caco General Mizrael Codio and ten others after they were captured in Fonds-Verrettes. In Hinche in January 1919, Captain Ernest Lavoie of the gendarmerie, a former United States Marine, allegedly ordered the killing of nineteen caco rebels according to American officers, though no charges were ever filed against him due to the fact that no physical evidence of the killing was ever presented.
The torture of Haitian rebels and the torture of Haitians who were suspected of rebelling against the United States was a common practice among the occupying Marines. Some of the methods of torture included the use of water cure, hanging prisoners by their genitals and ceps, which involved pushing both sides of the tibia with the butts of two guns.

1921–1927: Rif War: During the Rif War, Spanish forces used chemical weapons against Berber rebels and civilians in Morocco. These attacks marked the first widespread employment of gas warfare in the post-WWI era. The Spanish army indiscriminately used phosgene, diphosgene, chloropicrin and mustard gas against civilian populations, markets and rivers. Spain signed the Geneva Protocol in 1925, that prohibited chemical and biological warfare, while simultaneously employing these weapons across the Mediterranean.
According to Miguel Alonso, Alan Kramer and Javier Rodrigo in the book Fascist Warfare, 1922–1945: Aggression, Occupation, Annihilation: "Apart from deciding not to use chemical weapons, Franco's campaign to 'cleanse Spain' resembled that in Morocco: intelligence-gathering through torture, summary executions, forced labour, rape, and the sadistic killing of military prisoners."
Spanish mutilations of captured Moroccans were reported, including castration and severing heads, noses and ears, which were collected by Spanish legionnaries as war trophies and worn as necklaces or spiked on bayonets.
On August 9, 1921, the Massacre of Monte Arruit occurred, in which 2,000 soldiers of the Spanish Army were killed by Riffian forces after surrendering the Monte Arruit garrison near Al Aaroui following a 12-day siege.

1923–1932: Pacification of Libya: The Pacification of Libya resulted in mass deaths of the indigenous people in Cyrenaica by Italy. 80,000 or over a quarter of the indigenous people in Cyrenaica perished during the pacification.
100,000 Bedouin citizens were ethnically cleansed by expulsion from their land.