The United States' diplomatic security apparatus that operates today from Washington DC to Iraq and Afghanistan is uniquely massive. It is incomparable in its size, budget, degree of institutionalization, and level of sophistication when set against both other nations as well as its own humble origins in WWI. To understand why this is so, the first half of this chapter historically maps and causally explains how, and why, US diplomatic security has been transformed over the course of its modern hundred-year history. The second half provides an empirically rich study of the various roles and functions of the State Department's Bureau of Diplomatic Security and the US military units that protect the US diplomatic mission.