Fantastic movie. The singing girl went on to become Kubrick's wife.

I love, love, love this movie

She's singing an old war ballad called "The Faithful Hussar"

I really need to see this movie

That's... actually the last thing I expected to happen.

Wonderful.

Some explanation, though I doubt its really needed:

Europe itself was a melting pot, so everybody just about anywhere was at least a LITTLE Germanic.

The war happened, and they all forgot this aspect. When she starts singing they suddenly remember their German heritage and it jars the fuck out of them.