How did the communists devise and carry out their surprise attack?

It was 2:45 a.m., on Jan. 30, 1968—the day Vietnam celebrated the lunar new year—when Viet Cong troops raced through a three-foot hole they had blown in a wall protecting the United States Embassy in Saigon. Thus began the Tet Offensive, in which thousands of communist-backed Viet Cong fighters waged a series of major assaults on big cities, provincial hamlets and regional capitals across U.S.-backed South Vietnam—more than 100 locations in the first 24 hour alone. In addition to the bloody fighting in the Embassy courtyard, they waged fierce attacks on strategic targets such as the presidential palace, Saigon’s Tan Son Nhut Airport and the city of Hue, once a seat of emperors. In subsequent days, more waves of the offensive followed.