If this is an antique, go with what others have said about getting it evaluated by an expert and care for it accordingly.

Assuming this is a replica and you would rather it be polished, I've had good results with jewelers rouge and a buffing wheel. I'm not saying the experts will agree but sicne I don't do it for a living and don't want to put a lot of money into it, I'm working with what I have on hand. You have to be very careful so the metal doesn't get to hot but working in short bursts or across a large area I've not had any problems getting badly tarnished blades looking good again.