IN THE SPRING of 1954, the US Senate convened hearings at the instigation of Senator Joseph McCarthy to press his anticommunist investigations into the Department of the Army. The hearings were broadcast live on television, and the American public was able to witness firsthand the tactics McCarthy used to intimidate his foes. At a critical moment in the hearings, a key governmental witness, Army lawyer Joseph Welch, rose to defend one of the junior Army lawyers whose career, Welch alleged, McCarthy had destroyed. Welch turned to McCarthy and memorably intoned: "Let us not assassinate this lad further, senator. You have...