What were the lasting effects on the soldiers of World War 1?

shell shock

Millions of men came home with out a leg, an arm, or blind, or deaf, or mentally broken due to the things they had to live through in the trenches. Others had their lives cut short through the effects of poison gas, and injuries due to blast, with collapsed lungs.

Others came home whole in body, appearing normal, but with such serious nervous/mental conditions that they could not work, and were confined to mental hospitals for the rest of their lives. In addition, some were known to "See" combat zones in their normal areas. Ex: A man dove to the ground for cover when a boy rattled a stick on the fence. The veteran had thought it to be machine gun fire.

The majority were able to resume their lives, but with various amounts of disability.

It should be noted that most, especially on the Allied side, later believed the war to have been worthless.