I, luckily, have never been put into battle, but of all recent modern wars, World War I, had to be the worst for the soldiers, followed by The American Civil War.
WW2 was pretty bad too. I had a relative who fought his way from north Africa and north through Italy. He never got a shower from September till April. At night machines came in to drill holes in the ground for them to sleep in. He said some of them slept with their arm sticking up out of the hole thinking they might get wounded and go home. He said you can’t understand until you experience it.
They were all apparently completely unprepared for the horrors unleashed by the weapons they used.
It's been said that the reason population centers were targeted in WWII was due to the belief that the clamor by the people for an end to it all would hasten peace talks.