Corea was around from at least 5000 years ago. About the same age as China or a bit later. The first corean nation verified is called Chosun (they put 'Old' chosun to this nation to differenciate it with later 'chosun', because the later one used the name to claim its originality.) Note that I said 'verified'. There is a chinese record (a legend possiblely) of an ancient nation called Baedal, a corean nation. Coreans still call themselves 'the people of Baedal'. We don't know for sure if this nation existed. You know you can't take legends at a face value. But Since there is a record of it, many people believe that there WAS something, probably a nation built by coreans, but they are not sure what it might have been. (You know, far eastern history is so much distorted because of chinese's pride and japanese trying to hide their past.) Some theories even say that Coreans were actually who built china and when coreans lost in the power struggle, they were forced in the peninsula. Not like Dragon-Knight said in this forum, Goguryo comes much later than Chosun. It's not even second corean state. (of course it's not even third or fourth and so on... I'm not sure.)
This Chosun is believed to have ruled most part of china, korean peninsula, mongolia and southern part of present day russia and even more.
Corea lost much of it's power after that and had to be sort of under china for some time.