Looking for a cartoon movie where in the beginning of the movie, there is only two humans left and they kill each other in war, animals start to emerge from the forest and see a gas mask and helmets and guns laying on the ground but its finally silent, no sounds of war. I believe it was made pre 1990, I remember watching it in the early 90s but it was already an older movie then.

Maybe Wizards by Ralph Bakshi. Our movie expert, Quinn F, is sure to know.

This is just a short film, super old - but it's the first thing I thought of from your description... there is a scene with two final soldiers, gas masks, guns, and animals emerging from the forest... https://www.youtube.com/watch?v=8fAOIAGofDM

(first image of soldier is at 3 min...)

Wizards by Ralph Bakshi