A really bizarre thought I had went something like this:
Suppose “War of the Worlds” happened in the Star Trek history. So in the 1890s humanity learned that aliens existed, and the invasion fleet left lots of steampunk hardware around.
The Great War gets a whole lot nastier, with both sides trying to use reverse-engineered Martian weaponry. Fortunately, they can’t reverse-engineer the truly nasty stuff or figure out how to make most of what they’ve captured work. But think of the armies, going to war not with tanks but with the Martian tripods, impervious to artillery shells. Humanity very nearly bombs itself back into the Stone Age on a world-wide scale.
No, this is not a timeline of hope. Man’s inhumanity to his fellow man is well-documented. Warfare has become a nastier and more deadly affair over time for two reasons. One, weaponry itself has become more efficient at killing. Two, people can kill from a distance of continents; it’s one thing to be face to face with the one who might kill you, but when you can rain death on an opponent and never even see them or give them a second thought death becomes abstract rather than real. War has become dehumanized as the means of war have become more advanced.
If, if, humanity survives the Great War, we can hope that humanity will have turned a page. That humanity will have learned that its greatest threat to its continued existence on Planet Earth is itself.
If humanity survives but does not learn that lesson, imagine first contact with a race such as the Vulcans. Humanity has known for centuries that alien intelligences exist. Alien intelligences that tried to exterminate humanity. How will humanity respond? With the hand of friendship? Or the hand of war?