War crime
William Calley, former U.S. Army officer found guilty of murder for his role in the Mỹ Lai massacre.
General Tikka Khan, aka "Butcher of Bengal" was a notorious Pakistan Army General known for his war crimes in Bangladesh during the Indo-Pakistani War of 1971.
Ali Hassan Abd al-Majid al-Tikriti, more commonly known by his nickname "Chemical Ali", executed by post-Ba'athist Iraq for his leadership of the gassing of Kurdish villages during the Iran-Iraq War; also governor of illegally occupied Kuwait during the First Gulf War
Ratko Mladić, indicted for genocide amongst other violations of humanitarian law during the Bosnian War; he was captured in Serbia in May 2011 and was extradited to face trial in The Hague, wherein he was found guilty and sentenced to life in prison.
Joseph Kony, leader of the Lord's Resistance Army, guerrilla group which used to operate in Uganda.

Definition: War crimes are serious violations of the rules of customary and treaty law concerning international humanitarian law, criminal offenses for which there is individual responsibility.
Colloquial definitions of war crime include violations of established protections of the laws of war, but also include failures to adhere to norms of procedure and rules of battle, such as attacking those displaying a peaceful flag of truce, or using that same flag as a ruse to mount an attack on enemy troops. The use of chemical and biological weapons in warfare are also prohibited by numerous chemical arms control agreements and the Biological Weapons Convention. Wearing enemy uniforms or civilian clothes to infiltrate enemy lines for espionage or sabotage missions is a legitimate ruse of war, though fighting in combat or assassinating individuals behind enemy lines while so disguised is not, as it constitutes unlawful perfidy. Attacking enemy troops while they are being deployed by way of a parachute is not a war crime. Protocol I, Article 42 of the Geneva Conventions explicitly forbids attacking parachutists who eject from disabled aircraft and surrendering parachutists once landed. Article 30 of the 1907 Hague Convention IV – The Laws and Customs of War on Land explicitly forbids belligerents to punish enemy spies without previous trial.
The rule of war, also known as the Law of Armed Conflict, permits belligerents to engage in combat. A war crime occurs when superfluous injury or unnecessary suffering is inflicted upon an enemy.
War crimes also include such acts as mistreatment of prisoners of war or civilians. War crimes are sometimes part of instances of mass murder and genocide though these crimes are more broadly covered under international humanitarian law described as crimes against humanity. In 2008, the U.N. Security Council adopted Resolution 1820, which noted that "rape and other forms of sexual violence can constitute war crimes, crimes against humanity or a constitutive act with respect to genocide"; see also wartime sexual violence. In 2016, the International Criminal Court convicted someone of sexual violence for the first time; specifically, they added rape to a war crimes conviction of Congo Vice President Jean-Pierre Bemba Gombo.

War crimes also included deliberate attacks on citizens and property of neutral states, such as the Japanese attack on Pearl Harbor. As the attack on Pearl Harbor happened while the U.S. and Japan were at peace and without a just cause for self-defense, the attack was declared by the Tokyo Trials to go beyond justification of military necessity and therefore constituted a war crime.
War crimes are significant in international humanitarian law because it is an area where international tribunals such as the Nuremberg Trials and Tokyo Trials have been convened. Recent examples are the International Criminal Tribunal for the Former Yugoslavia and the International Criminal Tribunal for Rwanda, which were established by the UN Security Council acting under Chapter VIII of the UN Charter.
Under the Nuremberg Principles, war crimes are different from crimes against peace.  Crimes against peace include planning, preparing, initiating, or waging a war of aggression, or a war in violation of international treaties, agreements, or assurances.  Because the definition of a state of "war" may be debated, the term "war crime" itself has seen different usage under different systems of international and military law. It has some degree of application outside of what some may consider being a state of "war", but in areas where conflicts persist enough to constitute social instability.
The legalities of war have sometimes been accused of containing favoritism toward the winners ("Victor's justice"), as some controversies have not been ruled as war crimes. Some examples include the Allies' destruction of Axis cities during World War II, such as the firebombing of Dresden, the Operation Meetinghouse raid on Tokyo (the most destructive single bombing raid in history), and the atomic bombings of Hiroshima and Nagasaki.  In regard to the strategic bombing during World War II, there was no international treaty or instrument protecting a civilian population specifically from attack by aircraft, therefore the aerial attacks on civilians were not officially war crimes. The Allies at the trials in Nuremberg and Tokyo never prosecuted the Germans, including Luftwaffe commander-in-chief Hermann Göring, for the bombing raids on Warsaw, Rotterdam, and British cities during the Blitz as well as the indiscriminate attacks on Allied cities with V-1 flying bombs and V-2 rockets, nor the Japanese for the aerial attacks on crowded Chinese cities.
Controversy arose when the Allies re-designated German POWs (under the protection of the 1929 Geneva Convention on Prisoners of War) as Disarmed Enemy Forces (allegedly unprotected by the 1929 Geneva Convention on Prisoners of War), many of which were then used for forced labor such as clearing minefields. By December 1945, six months after the war had ended, it was estimated by French authorities that 2,000 German prisoners were still being killed or maimed each month in mine-clearing accidents. The wording of the 1949 Third Geneva Convention was intentionally altered from that of the 1929 convention so that soldiers who "fall into the power" following surrender or mass capitulation of an enemy are now protected as well as those taken prisoner in the course of fighting.

United Nations: The United Nations defines war crimes as described in Article 8 of the Rome statute, the treaty that established the International Criminal Court:

Grave breaches of the Geneva Conventions of 12 August 1949, namely, any of the following acts against persons or property protected under the provisions of the relevant Geneva Convention:

Other serious violations of the laws and customs applicable in international armed conflict, within the established framework of international law...
In the case of an armed conflict not of an international character, serious violations of article 3 common to the four Geneva Conventions of 12 August 1949...
Other serious violations of the laws and customs applicable in armed conflicts not of an international character, within the established framework of international law...

Legality of civilian casualties: Under the law of armed conflict (LOAC), the death of non-combatants is not necessarily a violation; there are many things to take into account. Civilians cannot be made the object of an attack, but the death/injury of civilians while conducting an attack on a military objective are governed under principles such as of proportionality and military necessity and can be permissible. Military necessity "permits the destruction of life of ... persons whose destruction is incidentally unavoidable by the armed conflicts of the war; ... it does not permit the killing of innocent inhabitants for purposes of revenge or the satisfaction of a lust to kill. The destruction of property to be lawful must be imperatively demanded by the necessities of war."
For example, conducting an operation on an ammunition depot or a terrorist training camp would not be prohibited because a farmer is plowing a field in the area; the farmer is not the object of attack and the operations would adhere to proportionality and military necessity. On the other hand, an extraordinary military advantage would be necessary to justify an operation posing risks of collateral death or injury to thousands of civilians. In "grayer" cases the legal question of whether the expected incidental harm is excessive may be very subjective. For this reason, States have chosen to apply a "clearly excessive" standard for determining whether a criminal violation has occurred.
When there is no justification for military action, such as civilians being made the object of attack, a proportionality analysis is unnecessary to conclude that the attack is unlawful.

International Criminal Tribunal for the former Yugoslavia: For aerial strikes, pilots generally have to rely on information supplied by external sources (headquarters, ground troops) that a specific position is in fact a military target. In the case of former Yugoslavia, NATO pilots hit a civilian object (the Chinese embassy in Belgrade) that was of no military significance, but the pilots had no idea of determining it aside from their orders. The committee ruled that "the aircrew involved in the attack should not be assigned any responsibility for the fact they were given the wrong target and that it is inappropriate to attempt to assign criminal responsibility for the incident to senior leaders because they were provided with wrong information by officials of another agency". The report also notes that "Much of the material submitted to the OTP consisted of reports that civilians had been killed, often inviting the conclusion to be drawn that crimes had therefore been committed. Collateral casualties to civilians and collateral damage to civilian objects can occur for a variety of reasons."

Rendulic Rule: The Rendulic Rule is a standard by which commanders are judged.
German General Lothar Rendulic was charged for ordering extensive destruction of civilian buildings and lands while retreating from a suspected enemy attack in what is called scorched earth policy for the military purpose of denying the use of ground for the enemy.