As you wait for a better answer, here is an older thread that discusses a similar topic.

It talks about literature during the Inter War period on how a future second conflict could have looked like.

Opinions vary from author to author, but a commonly shared fear was that of large-scale use of gas warfare and of the “knock-out blow”: a massive bombing strike that could level enemy cities and annhilate their defensive capabilities before any kind of land-based warfare would even begin.

"Prior to the 20th century" is the tough part here. "Prior to the atomic bomb" — sure. The technological marvels of World War I, like submarines and gas warfare and airplanes, inspired all sorts of dread and fantasies about destroying the world, and were part of what drove enthusiasm/dread for the atomic bomb in World War II (people who were young during WWI were in positions of power and influence in WWII, and these images drove them to pursue all sorts of wonder-weapons, looking for the new "edge" in warfare). But prior to World War I, there was a lot less focus on technology as the means of mass destruction, and a lot less discussion of the possibility of anthropogenic world destruction in general. The closest I have seen is a Parisian joke from the 1890s that Thomas Edison had a button to destroy the world in his workshop — and even that is pushing the "prior to the 20th century" requirement.

In general, technology was not the vector with which people thought about the apocalypse prior to the late industrial revolution. There was lots of apocalyptic thinking — often surrounding ideas from religion (e.g., the coming of the anti-Christ, Revelations, apocalyptic weather, divine plagues, etc.) — but technology as a distinct category isn't really part of it until the last century or so. The late industrial revolution really spotlit for people the dramatic, world-changing power of technology, and World War I is largely responsible for the trope of radically new technological weapons having the possibility of huge impacts.