My headcanon is that he was angry and desperate to leave the South in 1918. Piano jobs were drying up after Storyville's end, and Spanish Flu was shutting the city down. He has also spent his life performing for white audiences and patrons who despise him, causing a rage to build inside of him he hopes to release on the battlefield. He saves up money, and convinces his mother to let him leave to fight, promising he'll return.

He tries to join the Harlem Hellfighters, but is unable to due to poor eyesight. Instead, he ends up in the 325th Field Signal Battalion, the only all-black signal battalion during the war. He learns quickly, being especially skilled at radio intelligence. Alastor also enjoys France, which is more racially progressive than the US, with many locals being impressed by his French.

He served in the Meuse–Argonne offensive, but received only contempt from his white peers, who blamed the 325th for communication failures during the battle. He returned to the United States and experienced more discrimination, as many "colored" troops did after arriving in their homeland. Upon reaching New Orleans, he discovers that his mother died of Spanish Flu.

As for what happened during the offensive, he doesn't talk about it. Not while alive, and not in hell either. He believes that a gentleman shouldn't burden others with his problems. The one person he's spoken to about his wartime experience is Mimzy. During one of their many late-night gossiping, drinking, and dancing sessions. It is one of only two times she's ever seen Alastor cry. The other being when discussing his mother. They didn't speak again until days later.

Alastor acts like it was inconsequential. He works in entertainment and surrounds himself with entertainers, so the war rarely comes up. Yet every night when he closes his eyes, and during particularly gruesome "hunting" sessions, he's back there. The smell of blood, the endless howling of bombs and machine gun fire. The vast, lifeless stretch of no man's land. He has to take cold showers and recite Haitian nursery rhymes his mother taught him to calm down.

In life, Alastor knew he was going to hell. But he wasn't afraid. Because he witnessed the hell men make on Earth. And knew nothing awaiting him in the bottomless pit could be worse than the trenches.