The 1925 geneva protocol has been approved by the United States Senate, making it federal law at a level just below that of a Constitutional amendment. The one-page protocol, (see http://projects.sipri.se/cbw/docs/cbw-hist-geneva-eng.html), prohibits ". . . the use in war of asphyxiating, poisonous or other gases, and of all analogous liquids, materials or devices."
As I understand it, the reason for this phrasing was because a previous treaty (The Hague, ca. 1898) forbade the use of "poison or poisoned arms" in warfare. The earlier treaty, approved by the U. S. Senate in 1902, apparently was intended to refer to poisoned food or water, or to poisoned bullets, bayonets, etc. The Germans found a loophole in the Hague treaty and felt free to use poison gas in World War I.
However, there is good reason to consider photon particles in the same way as chlorine atoms or sarin molecules. Photons exert a pressure in a closed space and enter as reactants into chemical reactions. The gaseous pressure of light helps keep the sun and the stars from collapsing. The apparent differences between photonic and other gases can be reduced to the single fact that other gases are made of particles that travel below the speed of light.
Microwave photons can cause difficulty in breathing. Infrared or visible light photons can cause blindness. A laser or similar device can be held on a target like a hose, until the photons equilibrate, build up in and around the target (a soldier, missile, tank or ship), and thus gasify. In combat, a microwave beam most likely would be held on trapped or wounded enemy soldiers until the outer layers of their skin separated, leading to death by loss of fluids into the detached layers of skin.
By contrast, a nuclear detonation emits a single, sudden flux of photons, which cannot be made to gasify. Likewise, lead released as a poisonous cloud of dust may be contrasted with its delivery in a copper-jacketed bullet.
So, photons from a laser or maser are applied differently than those from more familiar weapons. The 1925 protocol only requires that a material or device be "analogous" to a gas.
If the United States deployed lasers or other photonic devices as part of a defense system, a case might be made in court (federal or international) that they were based on delivery of a harmful gas, thus engendering possible indictments for war crimes.
The U.S. Congress should bring up this issue with international authorities before deployment of any photonic system usable in war. The 1925 protocol does not apply to research or testing, so no breach of international law would have occurred yet.
It would seem wise to obtain a ruling before further spending.
John Michael Williams is a member of the Institute of Electrical and Electronic Engineers, the Optical sSociety of America and the American Physical Society.