Why might gas be a more effective weapon against soldiers in the trenches than those on the battlefield?

Gas would be more effective.

Explanation:

1. It is something that can be spread by different means, via bomb, pipe lines, or air filtration systems.

2. If gas were to be used, it would be unnecessary for soldiers to be committed into situations that might endanger them.

3. It also has more potential to have the element of suprise on it's side since it would unexpected and much more difficult to be prepared for and defend against.