Even before conventions, laws, and neutral parties who can rescue and treat the injured in the battlefield, the ancient world had some laws about how a war was designed to be fought. Some of the earliest examples include religious texts.
The Indian Mahabharata has a tale of two brothers who speak about war and the law of proportionality. The law states that forces cannot attack an objective if it results in massive destruction or death to civilians, regardless of the advantage gained.
The tale also speaks of just ways of war or fighting without causing unnecessary suffering or harming civilians. Having a Just cause is also a theme throughout the ancient world, and many nations needed a casus belli or reason to go to war, to prove that it was just.
For the Romans, the just cause was more than likely revenge on an invader or punishment for breaking a treaty, but in the early days,war was considered something that would make the Gods angry and required priests to come together and actually declare that it was okay.
In the later years of the Roman Empire, wars were raged for power, and the divine wrath was eventually pushed aside and ignored.
The laws of Just war began to be more fleshed out under Saint Augustine, who stated that Christians could go to war without fear of retribution when it was a war in defense of others or otherwise authorized by the church.
Saint Thomas Aquinas later laid out the terms of a just war, stating the following:
A war must be waged and authorized by the state or the church, then it must occur for the right reason. Warring for restoring lost land or punishing an evil act by a government or army are good reasons, but warring for the interests of a state is not.
Finally, war must be fought so people can live in peace, and the war must accelerate the peace process. Other people, such as the School of Salamanca, say that war must be fought for self-defense, or against someone who is about to attack only.
In addition, the war must meet the violence with violence but not exceed it. Simply stopping an invading army from crossing into territory would be just but raiding and pillaging their lands as invaders would be too much. The people must also not oppose a war, as it would be illegitimate to continue to wage a war if the people are tired of it.
Other views of just war
Various other nations and religious bodies during the ancient times had their own rules of what made a just war. While some civilizations simply ignored them and waged war for the sake of simple conquest or went to war and didn’t pursue all other diplomatic options first, many did follow these rules.
They eventually led to the forming of the Geneva Convention and led to a real groundwork for a true law of war that all nations could agree on to ensure the war and the treatment of the combatants was just and humane.