A more thorough explanation:

The Lieber Code is a set of rules and customs of warfare that was first developed by Francis Lieber during the American Revolution. It was formally adopted as law by Abraham Lincoln during the Civil War and used as the basis for the first codified international rules of law at The Hague Peace Conference of 1899.

The rules were extended and refined in another Hague convention in 1907, and became known as the Law of The Hague. The Law of The Hague is the first widely accepted body of international law of war, as approved by conventions in The Hague in 1899 and 1907.

The Law of The Hague set up procedures for mediation and arbitration of disputes to avoid war, and attempted to regulate the type and use of weapons in warfare.

For example, the Law of The Hague prohibits the use of chemical and biological weapons in warfare. This means that countries cannot use weapons that release poisonous gases or diseases to harm their enemies. Another example is that the Law of The Hague requires that prisoners of war be treated humanely. This means that they cannot be tortured or mistreated while in captivity.

These examples illustrate how the Lieber Code and Law of The Hague aim to regulate the conduct of war and protect civilians and soldiers from unnecessary harm.