the us army served late in world war 1, quite a few characters from WW1 served such as Richard Harrow, Al Capone and Jimmy Darmody. All three of which suffered injuries during the war
Richard Harrow - was a sharpshooter/marksman who therefore didn't spend any time in the trenches but suffered a serious facial injury in which most of the right side of his face was taken off (it is unclear how it happened) therefore he wears a mask over that side of his face to cover up the scaring, missing eye and missing side of his mouth, it is also possible that he suffered from PTSD or (post-traumatic stress disorder) as seen later in the seasons. He only talks about one incident during the war wear he tracked a German sniper for days and put a bullet right under his eye something he then did again back in america on request of Jimmy Darmody to retaliate for his prostitute friend who had suffered a facial scar as well.
Jimmy Darmody - was a run of the mill soldier in the trenches, he saids in the pilot that he went over (the trenches into no man's land) so many times that they felt he was a hero, when in reality it was because he had just given up caring about his own life, he also mentions that he once shot a German soldier who then got tangled in the barbed wire so badly that for three days Jimmy just listened to him cry out, but the soldier never let Jimmy put him out of his misery, during the latter stages of the war he caught a bit of shrapnel from a German grenade in his leg causing him to spend 8 months in the hospital and have 6 bolts in his femur. He see's his actions of the war as that of a murderers and see's that even though he is 22 he is going to go to hell for his actions. he often states to nucky that 'he isnt the same boy that left' suggesting that the war changed him heavily. Although his reasons for joining the army are often up to interpretation in some scenes its because he was finding school to hard, or was ashamed at having sex with his mum or that having a kid or getting married was a scary prospect for him.
Alfonso 'scar face' Capone - was a member of the lost battalion (that Jimmy jokes that they where so lost they thought they where in France but really where still in America.) he recieved two scars on his face from the war giving him his nickname. He however seems the least troubled by his time there and enjoys the heroism that comes with it.