UNLIKE many Western liberals, Stalin had no problem about recognising the moral equivalence of Soviet and Nazi totalitarianism. For example, he introduced Lavrentii Beria, his secret police chief, to Roosevelt at Yalta in 1945 as 'Our Himmler'. Rightly so. As head of the Soviet secret police, the NKVD, from 1938, Beria controlled an intelligence and concentration camp empire at least as large and evil as that run by Himmler.

This admirably cool and detailed book is the first serious biography of Beria, who was killed by his colleagues in the aftermath of Stalin's demise in 1953. Its author, a senior research analyst at the US Library of Congress, has been allowed unprecedented access to Soviet, Georgian and Communist sources in the former USSR.

Like Stalin, Beria came from Georgia and like Stalin, he was prepared to repress nationalist sentiments there with incredible brutality. Knight provides a masterly account of the manner in which the ambitious but ill-educated Georgian peasant lad, 18 when the Revolution took place in 1917, rose rapidly through the ranks of the newly formed Bolshevik secret police in Georgia to become deputy chairman aged 22, and, four years later, chairman.

Beria's first stroke of luck was to have been born at the right time. But his unique inspiration, Knight argues, was to use his base in the Georgian secret police to take over the party and government. In 1931 he became secretary of the Georgian Communist Party. Then he purged without mercy, filling former political posts with his thuggish policemen, creating the world's first police state. This fiefdom would survive as Beria's powerbase after he moved to Moscow in 1938 to take charge of the Soviet NKVD in the aftermath of the most bloody of Stalin's purges.

There, under the guise of winding down the Terror, Beria decimated the terror machine, replacing those purged with his own henchmen. He also ingratiated himself with his fellow-Georgian, Stalin, feeding the dictator's paranoia and encouraging his sadistic desire to demean his comrades. There are grotesque descriptions here of Stalin's regular drunken orgies at which people were forced to drink themselves senseless before being dragged out by guards.

Don't think, however, that Beria could not relax. He enjoyed taking part in the torture of top level prisoners and had a regular supply of young women kidnapped and brought to his dacha for him to rape. And he liked Hollywood movies.

When Stalin died in 1953 (almost certainly, Knight concludes, as a result of malevolent neglect by Beria and others) the secret policeman hoped to inherit Stalin's mantle - by acting as a liberal reformer. Whether this posture reflected a pragmatic decision that Stalinism could not survive in the long run, or a cynical, short-run, attempt to create a domestic constituency is unclear.

Whatever his motivation, Beria put an end to the renewed wave of terror, and to Stalin's anti-semitic campaign, which supposedly was to end with the mass deportation of the Soviet Jewry. He released more than 90 per cent of the 2.5 million political prisoners and introduced a more liberal nationalities policy. He wished to decentralise the command economy. He pushed East Germany to abandon Stalinism. But reluctance by the Stalinist Old Guard there sparked off an anti-Communist revolt in East Berlin, which was put down with heavy loss of life.

This destabilisation of the Soviet empire provided the pretext for Beria's more cautious rival, Khrushchev, to move against him. Beria was arrested and apparently shot months before his official secret trial and execution. The USSR was to survive for almost half a century longer.