One hundred years ago this Monday, after German troops marched into Belgium, Britain declared war and scarcely an hour later it sent its cable ship Alert into the English Channel. By dawn, amid heavy rain and wind, the crew had severed Germany's five most important Atlantic cables. For the duration of the war, Berlin's ability to communicate abroad, even with many of its embassies, was impaired.

Today we take for granted that information warfare ? whether the disruption of other nations' computer systems, the monitoring of citizens' telephone calls to detect terrorist threats or the use of social media to shape foreign attitudes ? is a key tool of national security. These measures, and the debates about their proper limits in a democracy, seem unprecedented because they are driven by new technologies. But virtually all our concerns about such tactics find their roots in the Great War, particularly in its first hours, when the Alert's hatchet-wielding crew began its work.