A PSYCHOPATHIC Buddhist warrior-king hardly sounds plausible in fiction, let alone in modern history. But the story of Freiherr Roman Nikolai Maximilian von Ungern-Sternberg, an Estonian-raised, ethnically German, tsarist officer, who became the last khan of Mongolia amid the chaos of the Russian civil war, has so many bizarre elements that the reader will soon believe almost anything.

James Palmer's account of his brutal and ill-starred life is elegant, waspish and evocative. It starts in Estonia (then a Russian colony) where Ungern, as the author calls him, was born into a harsh family of Baltic Germans, the feudal masters of the region. Ungern was an obnoxious child; he tried to strangle a neighbour's pet owl and was expelled from school. His career in the Russian military gained him no distinction, only hideous scars, from duelling. He was almost uniformly detested.

As the “whites” (monarchists) lost the civil war, Ungern's star rose. Other leaders were dead or in exile. In 1920 Ungern and a ragged band of men struck south into Mongolia, hoping, ludicrously, to use it as a base to reconquer Russia and restore the Romanov ancien régime (though in fact the Romanov princeling he backed had long ago been murdered by the Bolsheviks).

Unlike some Russian nationalists, Ungern liked the Mongols, seeing them as the “scourge of God” to punish a sinful Russia. The “yellow race”, he wrote, was “more vital and more capable of state-building” than the decadent whites. His beliefs were bolstered both by confused references to the more lurid bits of the Bible and by some firmly held but shakily understood Buddhist beliefs. He liked the ceremony and ritual of Buddhism, and particularly its respect for tradition: “its purity, the preservation of the old and correct order of things”, as Mr Palmer puts it.

Ungern's religious tolerance did not extend to Jews, towards whom he nursed a fanatical loathing. During his 130-day rule of Mongolia, he ordered that “Commissars, Communists, and Jews, together with their families, must be exterminated.”

Such personal savagery helped maintain a kind of discipline among his troops. Mr Palmer notes that he conquered a chunk of Mongolia through “sheer energy and desperation”. But this was unmatched by any ability to command his forces intelligently or to seek allies elsewhere in the region. Within a few months Ungern was defeated by the advancing Red Army, betrayed, captured, tried and shot. The Mongolian escapade marked the peak of his career, but it was also a dismal failure.

Mr Palmer has done a good job in disentangling the myths and horror stories that surround his much-reviled subject. The result is a confident account marred by occasional flaws: a penchant for mixed metaphors plus some fanciful lapses and minor factual errors. His accounts of Ungern's bestial cruelty are not for the squeamish. The best bit of “Bloody White Baron” is the way it brings the repulsive combination of tsarist-era absolutism and mysticism to life. The Bolsheviks ultimately proved far worse. But it is easy to see why so many people thought that any change from monsters like Ungern could only be for the better.