There wasn‘t anything modern, which was the reason we made the geneva convention rules after WW1. Thousands of soldiers were killed gruesomely by gas attacks in the trenches. In total 100.000 soldiers died due to gas attacks, while 1.000.000 were injured. To older pre-existing treaties

The first bilateral treaty banning the use of chemical weapons - poisoned bullets in this case - was signed in 1675 between France and the Holy Rome Empire, says Dr Joanna Kidd of Kings College London. "There's an argument that humans have always had revulsion to using poison in warfare," she says. "I suppose there are ways we are just accustomed to being killed." The Hague Convention of 1899 outlawed shells for the "diffusion of asphyxiating or deleterious gases" even before they had been used on the battlefield. (The very first use of gas, in the form of grenades rather than shells, is thought to have been carried out by the French in 1914 - the attack was so ineffectual that few even knew it had taken place until the war was over.)

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