Toss all your old ideas about what a battlefield looks like. Foxholes and mortar shells: out. In: container ports. Corporate server farms. Air terminals. Power grids. National security in the 21st century is an endlessly complex challenge. Some of the old weapons are precious little help against the new threats: you can’t use a battleship to pulverize computer malware. Add border security, economic security, and protection of personal privacy and individual rights, and the scope of the challenge calls for facing new facts.