The first two weaponized nerve agents,
GA/Tabun and
GB/Sarin, were both successfully synthesized before the Second World War by German scientists. As organophosphate nerve agents, they're essentially Raid for people. Sarin, for example, is 500 times more toxic than cyanide, and can be easily turned into an aerosol.
A single tiny drop worth on the skin is enough to kill a full grown adult in minutes. Inhaled, it takes significantly less. Thankfully, unlike the later V series agents, the G-series aren't persistent agents, and tend to break down in a matter of weeks. They're still effective as terrain denial weapons though, for much of that period.
IOTL, the Germans generally had the lead in chemical weapons technology, but avoided its battlefield use for fear that the Allies would produce much more of it. But is it plausible that nerve agents could end up getting used by either side during the war? If so, how? Would they be used as terrain denial weapons, offensive tactical weapons, or would they end up being used as strategic terror weapons, depopulating the industrial sectors of cities?