Egypt used chemical weapons in the 1960s; no one did anything then either

Arabs, allied with Russia, kill hundreds of other Arabs with chemical weapons. A cover-up of chemical weapons use and angry denials, allowing Western nations stand by and do nothing for political reasons.

In 1967.

From Chemical and Biological Warfare: A Reference Handbook, by Albert J. Mauroni (2007):

The Yemeni Civil War (1962-1970) pitted the Yemeni royalists of the deposed imam against the Yemen republican forces in North Yemen, with Saudi Arabia and Jordan supporting the royalists and Egypt supporting the republican forces. This war was fought for five years until the two forces reached a stalemate in 1967. Although there had been occasional mentions of Egyptian military employment of mustard agent—filled bombs between 1963 and 1966, in 1967 these attacks became more frequent. International journalists began reporting that Ilyushin heavy bombers were dropping mustard-filled and phosgene-filled bombs on cities and rebel bases.

In January 1967, a gas attack near Sada killed more than 125 people. In May, two villages suffered 75 casualties from phosgene-filled bombs. Between 1967 and 1968, it is estimated that more than 1,000 Yemeni were killed as a result of exposure to CW agents. An International Red Cross mission sent doctors to assist the wounded, and the doctors testified to what they saw. Al-though they were careful to clarify that they did not see any evidence of actual attacks taking place, the signs and symptoms of the victims included burning eyes and trachea, pulmonary edema, internal thorax pain, extreme fatigue, and anorexia. Their findings were that in all probability these victims had inhaled toxic gases (Cookson and Nottingham 1969).The doctors were reluctant to identify the specific chemical warfare agents used, in part because they wanted to retain their neutrality and access to war victims. Although it appeared conclusive that mustard and phosgene had been used, a few cases suggested the use of nerve agent—filled bombs as well. The problem was how to prove the use of chemical warfare agents and who was responsible for using them. Because there were no arms control experts assigned to monitor or investigate these attacks, there was very little evidence other than eyewitness accounts from civilians and what could have been propaganda from the royalists. Although bodies and samples were sent to Saudi Arabia for more study, again, it was difficult to accuse any specific nation. Egypt claimed it had not used chemical weapons in Yemen, and, according to some sources, this may be true if Soviet air crews were manning the Egyptian-marked bombers that attacked those cities.

When Saudi Arabia and the royalists tried to get the United Nations to investigate, the UN's secretary general, U Thant, declined. On March 1, 1967, he stated that he was "powerless" to investigate the issue, and that the facts were in sharp dispute. Although he almost certainly knew exactly what was going on in Yemen, he had made a political decision to stay out of the affair. The U.S. government, occupied with answering criticisms about the use of Agent Orange and riot control agents in Vietnam, chose not to get involved. The U.S. military decided that the chemical warfare attacks were an aberration and not reflective of any requirement to worry about future chemical warfare attacks (and in 1972, chose to disestablish the Chemical Corps). The United Kingdom was attempting to reestablish relations with Egypt at that time, so it chose not to say anything publicly against Egypt or Soviet affairs in the Middle East (Seagrave 1981, 124-125). The incident became a political nonevent, fodder for the arms control community but not much else.

This incident teaches several interesting lessons. The first is the failure of the world's nations to react against the use of chemical weapons against civilians and military forces that were not similarly armed. This was not a clear violation of the Geneva Protocol of 1925, since Egypt was not then (and still is not) a signatory of the Geneva Protocol, unless it could be proven that Soviet crews were in those bombers. The reason that some military analysts believe there were Soviet crews in the bombers was twofold: First, they do not believe that the Soviet Union would have allowed Egypt to own or employ chemical weapons in 1967, Egypt having just started its interest in an offensive CW program. Second, the bombers dropped their munitions upwind of their targets for maximum effect, and in some cases, MiG fighter planes came back to drop high explosives or napalm on and near the targets to reduce or eliminate the evidence. These same tactics were seen years later when the Soviet air force attacked Afghani villages with chemical weapons. Because the attacks occurred in such remote locations and because post-mortem examinations took place days or weeks later, it was very difficult to directly attribute the cause of death to the bombing attacks. 

This was the first instance of Arabs attacking Arabs with chemical weapons.

The second was the Iran-Iraq War, where some 45,000 are believed to have been killed by chemical weapons.