WarCrimes p.3 (Patreon)

I’m going to be intentionally vague about who exactly is fighting and over what because all we really know is a) Surge is canonically a foreigner (originally American but retconned to Unovan when they decided real world countries don’t exist) b) people in Kanto served under him and he’s generally a well-liked personality c) there was SOME kind of war that happened with no elaboration beyond “Surge fought in it, they used pokemon on the battlefield, and guns exist in this universe”

I figure the easiest answer for the sake of this story is just that he immigrated here sometime when he was younger and enlisted to participate in a war Kanto fought against some other unnamed nation.