Warmatrix

Throughout history, technological, industrial and social developments have made war progressively more destructive and 'total', increasing its impact on society. During the Second Industrial Revolution war started to become so terrible that several nations banded together to set down rules and laws for it to curb its excesses. Though enforcement of these rules is often lacking, they have prevented much looting, rape, torture, starvation, mass killings and other war crimes.
Before the late 19th century CE, there were no formal rules of war, only informal ones, like a sense of honor and/or pity. In 1874 CE an attempt was made to set up laws of war in the form of the Brussels Declaration, which borrowed heavily from the Lieber Code, drawn up by US president Abraham Lincoln during the American Civil war. However few countries ratified it.
The first serious international effort to limit war was the Hague Convention of 1899 CE. The convention was an idea of tsar Nicholas II of Russia. It too copied many rules from the Lieber Code, though omitted much of the reasoning for them. It set down rules for the limitations of martial law; protection of civilians and property; how to handle prisoners of war, partisans and spies; conditions of armistices and much more. All major powers of the time ratified the treaty, but the USA excluded some additional declarations that prohibited the use of weapons like airborne bombs, poison gas and dum dum bullets. An international Permanent Court of Arbitration was established that remains in existence up to present day.
In 1904 CE US president Theodore Roosevelt initiated a second convention, which took place three years later. It expanded the rules of the first convention, adding treaties that covered declarations of war and several on naval warfare. More countries ratified the treaties.
During World War I the treaties were put to the test and on several occasions violated, for instance by the use of mustard gas on the battlefields of Europe. This triggered the Geneva protocol of 1925 CE, which banned the use of chemical and biological weapons.
Parallel to the Hague Conventions, Geneva Conventions took place in 1864 CE, 1906 CE and 1929 CE. Were the Hague Conventions handled what is allowed in war, the Geneva Conventions discerned between military personnel and civilians and set down rules to protect both. The first focused on the treatment of the wounded and acknowledged the International Red Cross, which had been established a year earlier. The second treated naval forces and the third the treatment of prisoners of war.
Evolving warfare and further transgressions of the Hague Convention treaties during World War II called for an update. In 1949 CE all earlier treaties were revised and bundled in the Fourth Geneva Convention, usually referred to as the Geneva Convention. In 1977 CE it was extended by a few additional protocols. Today, nearly 200 countries all over the world have ratified it, though some with reservations and limitations.
Judgement in disputes over (transgressions of) the Geneva Convention are handled by the United Nations Security Council, established four years before the convention. But enforcement of these judgements is left to member states and does not always take place. The convention is further hampered by the blurring of the distinction between civilians and armed forces. Despite all this, it still forms the baseline for rules of war.