There are many subtle and often-unseen ways that international law is integrated in our domestic systems and institutions. Case in point: I have worked on some cases involving U.S. servicemembers, and noticed they are each issued a “Geneva Convention I.D. Card”.
In compliance with the fourth Geneva Convention of 1949, the U.S. requires all employees of the Department of Defense, from combat troops to civilian staff, to carry a card such as those pictured below (an older and newer sample); the card must include the bearer’s name, ranks, affiliation, and other biographical details.
The idea is to facilitate the fair and ethical treatment of prisoners of war by allowing whoever has captured them to determine whether they’re part of the regular armed forces subject to protection, are civilians rather troops, etc.; it also helps provide a clear record of who has been killed, imprisoned, or missing.
The idea of treating captured enemies fairly sounds quaint and absurd; but the drafters of the Geneva Conventions, in a display of calculated pragmatism, recognized that ending war was going to be a longer and more difficult goal than trying to restraint its worst impulses.
If nothing else, a Geneva Convention I.D. Card serves a practical purpose, and like most areas of international law we have chosen to abide by, it is the practical benefits that often motivate us; see similar treaties that sent universal standards for mail, shipping, airlines, passports, etc.