The reciprocity between the Provisional Irish Republican Army (PIRA) and the British authorities is analyzed as a case in point to illustrate how each side can adapt or reciprocate to each move or countermove. The goals, organizational structure, recruitment, finance, weapons procurement, and community support of the PIRA are examined over a 20-year period. In each category the PIRA has systematically evolved in reaction to counterterrorist policies pursued by the authorities. Likewise, the authorities have learned and adopted new counterterrorist techniques. Rather than militarily defeating one another, which is the goal of both terrorist and counterterrorist, the two sides seem locked in a ritualistic dance of death that can never end because the adaptive capacities of each appear inexhaustible. 1 table, 50 references. (Author abstract)