Cyanides were used as poisons long before Hydrogen Cyanide.  Pure cyanogen processes were developed in Germany in the 1800’s.  The French military first investigated cyanides for their military potential.  They discovered Hydrogen Cyanide, and it’s lethal capabilities, in 1916.  In WWI the French used 4,000 tons of HCN with little effect.  Not long after WWI it was discovered that HCN could be much more lethal if it was used as a spray and absorbed into the skin through direct contact, rather than through breathing HCN fumes.