On January 22, 1944, the Allies landed at Anzio, 35 miles from Rome; not until the 4th of June did they enter the city. What intervened, on the Beachhead and in the mountains, was probably the grimmest fighting of the war--akin to the static, decimating trench warfare of World War I. In restive occupied Rome, meanwhile, tension mounted to the popular outcry commemorated in Rosselini's Open City and the massacre of hostages in the Ardeatine Caves. From the many separate chronicles of these events (in many languages), Trevelyan-a 20-year-old junior officer at Anzio--has fashioned a stirring cross-cut epic (somewhat overdramatized, perhaps, in its Roman segments) that also carefully scrutinizes the several controversial episodes. Blame for failure to break out of the Beachhead remains with dithering American general John Lucas; but his two superiors--the self-important Mark Clark and the lethargic Alexander--are faulted, too, for their reluctance to relieve him. Athwart the German Gustav Line lay the ancient monastery of Monte Cassino--whose bombing by the Allies became a German propaganda coup when the abbot, rushed to Rome, testified that (as claimed) no Germans were within its precincts. On that disputed decision (which he does not condone), Trevelyan is masterful: ""Harold Nicolson announced that he would rather his son died than let the Monastery be destroyed""; ""The tired infantrymen, battling for their lives near its slopes, were to cry for joy as bomb after bomb crumbled it into dust."" And, from newly released Vatican documents, he reveals that German emplacements were in the outskirts of the Monastery. Still, its destruction permitted the Germans to occupy the ruins, thereby strengthening their defensive line. Within Rome, unoccupied Vatican City and its associated monasteries and convents provided refuge for escaped Allied prisoners, hunted Jews, and antifascist resisters--with the Pope's backing. Yet his ""silences"" remain clouded; the evidence--was it tactical accommodation or overriding anticommunism?--is still, to Trevelyan, inconclusive. Nor does he scant the Germans--their ""stamina and bravery"" at Cassino; their withdrawal from Rome, ""mostly very young and in rags,"" some weeping, some singing defiantly. A book absorbing in its detail, penetrating in its judgments, distinctive in its sweep.