holding a society together. Subverting or killing these leaders is likely to be the cheapest way to make an invaded country submit. When the Norman French invaded England in the eleventh century, they killed or impoverished most of the indigenous landowners; when the Turks, and then the Mongols, invaded India, they killed both landowners and priests; when England suppressed the Scottish highlands after the 1745 uprising, landowners were induced to move to Edinburgh or London; and in many of the dreadful events of the last century, rulers targeted the elite (Russian kulaks, Polish officers, Tutsi schoolteachers, …).

Moving from politics to commerce, the music industry spends a lot of money attempting to disrupt peer-to-peer file-sharing networks. Techniques range from technical attacks to aggressive litigation against individuals believed to have been running major nodes.

Networks of personal contacts are important in other applications too. In public health, for example, it often happens that a small number of individuals account for much of the transmission of a disease. Thus Senegal has been more effective at tackling the spread of HIV/AIDS than other African countries, as they targeted prostitutes [@Thom03]. In fact, interest in social networks has grown greatly over the last 15 