Does anyone know who wrote the first Equestria At War story?

Maybe someone who's been in the EAW community longer (or someone in the EAW forum) has a better answer, but I don't think there's really a "first" EAW story. There have been World War-themed fics since the very beginning of the fandom, and the mod sort of grew out of that.

If you really need a name, Lil Penpusher might be the best answer, since AFAIK they wrote a lot of the early story events and lore.

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In that case, can I please get a link to his user profile?

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Lil Penpusher

Just so you know, I've never read their work, I just remember they were involved with EAW story writing. And they seem to have a lot of stories set in an EAW or EAW-like setting.