In the post-Cold War era, US relations with the two Korean states – the Republic of Korea (ROK) and the Democratic People's Republic of Korea (DPRK) – are undergoing drastic and profound changes. The 9/11 terrorist attacks on American cities were a watershed in US–Korea relations. Even in the post-Cold War era, the Korean peninsula remains an area of crucial importance in overall US geo-strategic and geo-economic calculations. In 1948, two separate Korean states were created – the ROK in the south under the auspices of the US and the DPRK in the north with the Soviet red army's support. Presidents R. Reagan and G. H. Bush engaged North Korea in a very limited manner, which came to be known as 'smile diplomacy.' The Bush– Roh Moo-Hyun summit in May 2003 in Washington, DC failed to narrow their differences over North Korea's nuclear crisis. The chapter also presents an overview of the key concepts discussed in this book.