Which development best illustrates why world was 1 is classified as a foal war

The best choice you have is Chlorine gas. The results were so bad from it that after the war, the Geneva Protocol made the use of Chlorine Gas an international crime.

Any army using it would be held accountable for its use. It has terrible properties. Death is by asphyxiation.

Aerial battles where another example. At first it was a gentleman's war off the ground. Damaged planes were allowed to land if they could. There were rules. Later on, such mercy by either side was not given.