From Unreconstructed rebeL;

We whupped the best they sent us, and we whupped 'em fair an true
We whupped thier German immigrants and they Eyetalians too
We whupped Frogs and Square Heads and all their furrin might
But when the wen and got the Micks, we knew we'd got a fight.


I know Eyetalians, and micks (ireland), but what are frogs? (French? Unlikely to have fought for the union?) .And what are square heads? What does 'furrin' mean in this situation, and what does 'wen' mean?