Given the colossal egos involved, I think that all of the leaders, with certain foreknowledge, would have still embarked on war; certain that THIS time they would win. After all, there was honor and chivalry at stake. They all thought of war as something noble. They had a mindset and world view that is hard to fathom today.
There was a wonderful series on YouTube called
“The Great War,” that followed the war week by week.
https://www.youtube.com/watch?v=FrCOKVnBxBM
It was the meeting of modern technology and antiquated Generals and monarchs who cared not a whit about the citizenry. The bloody result was predictable. I doubt they would have stopped even knowing. The proof is that they didn’t stop when it did get nightmarish.
Thry merely searched the world for more bodies from people they cared even less about. That’s where we came in.
It’s laughable that it was called the war to make the world safe for democracy.