The Democratic People's Republic of Korea (DPRK), or North Korea, occupies the northern half of the Korean Peninsula, which juts out between the Yellow Sea (also known as the Korea Bay) and the Sea of Japan. To the north, the DPRK shares a 1,416-kilometer (880-mile) border—along the Yalu and Tumen Rivers—with the People's Republic of China, and a very short 19-kilometer (12-mile) border with Russia. North Korea's most conspicuous neighbor, however, is South Korea, which sits across a 238-kilometer (148-mile) border running from east to west. The border between North and South Korea, it is important to note, is no ordinary one. Known as the Demilitarized Zone, or DMZ, it is one of the most intensely guarded and heavily militarized borders in the world. Almost no human activity and development exists within the 4-kilometer- (2.5-mile-) wide zone, but nearly 2 million military personnel and a...