What is the main conflict in A Clash of Kings by George R.R. Martin?

A Clash of Kings is a fantasy novel by George R.R. Martin, which concerns the struggle for the throne of the Seven Kingdoms. There are several claimants to the throne, including Joffrey Baratheon, the current king, Stannis Baratheon, Renly Baratheon, Robb Stark, Daenerys Targaryen, and Balon Greyjoy.