1868: "The Declaration of St. Petersburg of 1868, for instance, prohibited
the “employment of arms which uselessly aggravate the sufferings of disabled men or
render their death inevitable.”
1899,1907: Landmark events such as the Hague Conventions of 1899
and 1907 sought to establish rules and procedures that would eliminate unnecessary
suffering by combatants and protect—to the extent possible—noncombatants and
1925: Similarly, the Geneva Protocol repudiated poison gas as a
legitimate weapon of war after its disastrous use in World War I.2
1929: The Third Geneva Convention incorporated a separate convention on the rights and treatment of
prisoners of war. Still other international conventions banned certain activities in time
of military occupation. Although many of these conventions were sometimes ambiguous
and not all governments signed or ratified them, by World War II it became clear that
warfare was no longer an unregulated affair.
1969: Banned in the US by Nixon.
1977: US (finally ratified) the 1925 Geneva convention.
2009: Obama announces an end to a program he didn't know about but did because of Wikileaks.
Professor Harris believed the final decision for the quid pro
quo was approved not only by Maj. Gen. Charles A. Willoughby (G-2) and Gen. Douglas
MacArthur in Tokyo, but by their superiors in Washington. What happened to the data
produced by Unit 731 remains largely unanswered. As Kondo Shōji has noted, several
key documents outlining U.S. acquisition of Unit 731 research data have not yet been
located.  " - Researching Japanese War Crimes
The US ratified the 1925 Geneva convention on bioweapons and poisons in 1977.
One wonders if they've spent more money on causing disease than curing disease. By way of the special relationship, the UK appears to be equally complicit.
After WWII the British took the G series gases from Nazi research and improved them into the Vx series gases. Note that Hitler was too afraid to to ever use the stuff he had made, which was based on a decades old insecticide. These organophosphates are still in use today as insecticides by the way.
The UK then traded Vx series gases to the US for nuclear technology. Britain remains the world leader in this technology. Note that the UK and UK used biological weapons on each other in 1776 and they were also used in the civil war. Back then smallpox and anthrax were the weapons of choice, the British fascination with Anthrax led to its development in a weopanized form in WWII. It is still is development as of this writing, a government lab in the US sent out "inert" samples of the bug to over a hundred other labs but claim to have mistakenly sent live anthrax instead.
On a personal note, I lived are one point down the street from Farley Mowat's home. Next door is an armory that in 1953 let anthrax infected misquotes escape.
 "J.I.S. Groups and Japanese National Revival, 11 May 1951, NA, RG 263, entry ZZ-18, CIA
Name File, box 6, folder: Hattori Takushiro, Vol. I."