It's 200 years old, but what is Italy's carabinieri?

I've always been puzzled by the fact that Italy has two police forces, although Italians don't seem to find it strange at all.

If you ask "why two?" they'll tell you, by way of unsatisfactory explanation, the polizia are the regular state police while the carabinieri are part of the army.

The real reason is a quirk of history.