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I think Americans would have to be Treemen, it took them a long time to get involved in the fighting because they had to arrange an Entmoot and we all know how long they take!!! I dunno, just a bit of fun. Perhaps Wood Elves?
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Wood Golems?

Slow to move? Or maybe Mud Golems?

Wood Elves would work.

More akin to the AD&D Wild Elves?

Actually they would not be missed for most early to mid war based on actual battles/campaigns scenarios because... they weren't there (like you said.) If it wasn't for unrestricted submarine warfare and Woodrow Wilson (oops, this enters the murky realms of American Politics, strike everything after "submarine warfare") I doubt we would have shown up for the party.
Apocryphal stories exist of Westerners/Southerners joining up thinking we were going to fight the "English" again.

How true those stories are is debatable I believe.
Gracias,
Glenn