A Soldiers Story question.

A movie I have admired for some time, watched it again today and still enjoyed it.

But there is a line of dialogue in the barracks between Denzil Washington and Adolphe Caesar, where Washington says...

"Where you from...England?"

And the reaction from both Caesar and the rest of the soldiers is so much that they all take a step back, and Caesar is clearly pissed.

And speaking as an English person I never really got the reaction to that line.

What am I not getting?