During the world wars, biological/chemical warfare was that period's equivalent of the atomic bombs. It laid waste to entire armies and left survivors either permanently injured or close to death. When it was first used in 1914/1915, no one foresaw the damage and suffering it would cause, so in World War II, there was a unofficial policy equivalent to the policy of Mutually Assured Destruction. One side feared using biological and chemical warfare because they feared retaliation from the other side.
But, that didn't stop both sides from stockpiling their own supplies of biological and chemical agents for use against their enemies. Though, thankfully, no one was crazy enough to use chemical/biological warfare on a large scale in Europe or on the Pacific, or at least on a scale where the other side decided to retaliate using similar weapons.
But, let's say push comes to shove and one side decides to take the gloves off. Who is more likely to be the side that starts using biological or chemical warfare during World War II, on either front? Bonus points if you can even mention a specific country and a scenario where biological/chemical weapons are first unleashed.
PS: Yes, I know Japan was using biological and chemical warfare in China during the Second Sino-Japanese War, but since that war started before World War II, that doesn't count.