The idea that warfare should be conducted honourably and within customary or codified rules certainly predates the 20th century. The modern classification of "war crimes" was codified in the Geneva Conventions which were signed in 1864.

Various societies had different rules for warfare in the past, many of which we wouldn't really recognize as consistent with our modern view of human rights. The Bible contains a prohibition on cutting down fruit trees during a siege for example, and various eras and cultures had conventions on the extent to which a conqueror could sack a city that surrendered vs. a city that didn't surrender.