How can an entire country be taken offline?
The answer depends on the country. In countries such as Syria, which went offline last week, Internet connectivity is so centralized that unplugging a few routers could take nearly everyone offline. By contrast, blacking out the United States probably requires a virtually endless game of Whac-A-Mole to shut down thousands of independent providers.
Sources: Marty Lindner, principal engineer with the CERT Program of the Software Engineering Institute at Carnegie Mellon University; James Cowie, chief technical officer of Renesys; CloudFlare. Bonnie Berkowitz and Bill Webster/The Washington Post.