fter Benito Mussolini was deposed in July 1943, the new Italian government headed by Marshal Pietro Badoglio began secret surrender negotiations with the Allies. In mid-August, hoping to spare Rome from destruction, the new government declared it an ''open city'' -- in effect, a demilitarized zone free from military activity by the warring antagonists.

The announcement of a general armistice was broadcast by Eisenhower on Sept. 8, and the Allied invasions at Salerno and Taranto on Sept. 9 marked the opening shots of the Italian campaign, the longest and costliest battles fought in the West during World War II. By invading Italy the Allies hoped to compel Hitler to maintain a large concentration of German divisions there instead of shifting them to northern France to help repel the forthcoming cross-Channel assault in 1944.

Both sides coveted Rome. Churchill was insistent that Rome fall by the end of 1943, and pressed Eisenhower and the Allied ground commander in chief in Italy, Field Marshal Sir Harold Alexander, to bring it about. Although Rome's military importance was primarily its airfields and vast road and rail network, it was a political and psychological prize beyond compare. Its capture would signal that Berlin, already under round-the-clock attack by British and American bombers, was next.

A stunned and infuriated Hitler vowed draconian retribution for what he deemed Italy's perfidious betrayal. As German reinforcements poured into Italy, Hitler's onetime ally became an enemy against whom a series of brutal reprisals was carried out. The notion that Rome was an open city lasted less than a month, as German jackboots echoed in its streets and Allied bombs again began falling.

However, Rome did not fall to the Allies in 1943. Instead, Salerno became the first of a series of grinding battles of attrition fought mostly along the spine of Italy against a skillful enemy. By the end of 1943 the Allies were mired in a deadly stalemate around Cassino, and the liberation of Rome seemed a fantasy.

In October 1943, the Germans shipped more than 1,000 members of Rome's Jewish community to Auschwitz, where more than 800 perished. When Italian partisans began guerrilla operations against the Germans on the streets of Rome, the once distant war suddenly came to the heart of one of civilization's oldest and most beautiful cities.

Thrust into this volatile mix were Pope Pius XII and the Vatican. Alarmed to the point of obsession by what he believed was the threat posed by the Soviet Union, Pius XII was intent on preserving the status quo of the Vatican State and preventing the spread of Communism, an evil he deemed even worse than Nazism. As German outrages continued, some of them within sight of the Vatican, there was no condemnation or protest from the pope, only silence.

So began the fierce battle for Rome, the subject of Robert Katz's gripping new book. Drawing on a wealth of interviews with participants and the recent release of previously secret documents from Italian, German, Vatican, O.S.S. and C.I.A. archives, Katz relates the tragic story of a great city held hostage to the fortunes of war and the contradictory designs of four mismatched parties. There were the Allies, ''trying to capture Rome as their first shining prize of war but discovering impregnable opposition''; the Germans, whose aim was to push back the Allies while holding Rome as a staging ground to resupply their front lines; the Vatican, ''trying to bring the West and the Germans to terms to save the world from 'Communism' and to save Rome and Vatican City from physical destruction''; and, finally, the diverse resistance groups of the left and right, whose common goal was to make life hell for the Germans.

The nine months between the German occupation in September 1943 and the city's liberation in June 1944 form the core of ''The Battle for Rome.'' At the book's center is Pius XII, whose inaction and failure to condemn German atrocities, particularly the massacre of 335 innocent Italians in the Ardeatine Caves in March 1944 -- a deadly German reprisal for a highly successful partisan attack on a column of SS police -- remain an object of fierce controversy to this day.

Katz, the author of several books on Italy and other subjects, skillfully weaves into his narrative the experiences of a large, fascinating cast of characters -- ordinary Roman citizens, informants, craven opportunists, spies and double agents, and some Germans who risked death in an effort to save Rome's Jews. During those terrible months Rome became a hotbed of murder, intrigue, betrayal and the bravery of resistance fighters who were relentlessly hunted by the Gestapo.

The unseemly race between British and American forces to be the first to reach Rome was the culmination of a series of blunders by the Allies, including a disastrous setback on the Rapido River, and at Anzio, where they were nearly thrown back into the sea. It was no small irony that a mere two days after headlines announced the fall of Rome on June 4, 1944, the D-Day landings in Normandy completely overshadowed the city's liberation.

In an addendum, Katz recounts his decade of legal troubles with an unforgiving Vatican and the heirs of Pius XII for having asserted in his 1967 book, ''Death in Rome,'' and in his screenplay for a subsequent film based on the book, that the pope had known in advance of the massacre of the Ardeatine Caves but had remained silent and failed to intervene with the Germans. In 1974, the first of five penal proceedings against Katz and two co-defendants was initiated and resulted in guilty verdicts and jail sentences. After numerous appeals and retrials, Italy's Supreme Court eventually dismissed the case.

Undeterred by his earlier legal entanglements, Katz draws on persuasive new evidence in ''The Battle for Rome'' to condemn unequivocally the ''Faustian pact'' between the Vatican and the Germans, in which the pope remained silent throughout ''the whole range of Nazi and Fascist brutality in Rome.'' His book is a poignant, dramatic and definitive account of a tragic time, and it is likely to reopen the longstanding controversy over the role of the Vatican and Pope Pius XII during World War II.