How we periodize history matters, be it for teaching or in research. So where does one draw the line? The beginning or end of a major conflict is common, but wars rarely end with a clean finish, and the human consequences are felt for years and decades after the cessation of hostilities. For historians of the United States, the transition from one presidential administration to another offers a fairly clean break; it entails personnel changes at the top levels from the president and department secretaries on down. Furthermore, new administrations have a major stake in sharpening this delineation as much as possible: out with the old, in with the new.
In reality, we know that the line is a great deal blurrier than partisan boosters and spin doctors would have us believe. Aaron Donaghy's The Second Cold War takes on what is commonly perceived as a particularly sharp break in...