It is a frivolous, almost crass question arising from the greatest carnage endured and prosecuted by mankind: who was the most important single figure in the Second World War?

It is an inquiry to provoke debate rather than a definitive answer, but Georgy Konstantinovich Zhukov is a persuasive choice. Most historians agree Hitler's army was conclusively defeated in Russia rather than on the Western Front, and it was Marshal Zhukov who was at the head of Russia's move from desperate defence to unforgiving, awful attack.

He was key to both defence of Leningrad, where a siege inflicted dreadful losses among the civilians, and Stalingrad where, famously, all animals fled to leave a hellish battlefield inhabited only by human beings. Zhukov, too, was key to the victory at Kursk that was, perhaps, the most crucial set-piece battle of the war, and he led the Red Army into Berlin.

Yet this astonishing record and these campaign medals did not protect him from the capricious wrath of Stalin and the deadly envy of his rivals. In his memoirs, the man who broke the German Army wrote of a time just three years after the end of the Great Patriotic War: "I feared arrest every day and I had a bag ready with my underwear in it."

How Zhukov found himself in this predicament heavily influences Roberts's sober, judicious assessment of an extraordinary soldier. The paranoia, jealousy and ego that led to Zhukov twice being stripped of honour and sent into internal exile makes for a fascinating story but it also impacts heavily on much of the prodigious research carried out by Roberts, Professor of History at University College Cork.

Every source, every diary entry, every letter has to be read in context. The praise of Stalin and Zhukov, for example, was once obligatory under pain of death. The denunciation of the great general also became mandatory as he fell from favour under Stalin and Khrushchev. Roberts, of course, is aware of this factor and is meticulous in detailing the background but it means Zhukov defies a strong, definitive analysis in terms of his personality.

His career as a soldier is easier to evaluate. Roberts draws Zhukov in blunt, bold strokes, which suit a man born to peasant stock in 1896 and who, less than half a century later had become a candidate for the most influential man in the Second World War. Zhukov's words, particularly in his memoirs, were regularly artic-ulated after a careful consideration of the politics of the time. However, some sentences stand out as the authentic representation of the peasant boy who became a world figure. "A difficult life is life's best school," he said, and this accurately reflects not only his upbringing but his ruthless approach to both existence and war.

Zhukov has been portrayed by some as a monster but it is perhaps more fair to say that he bent his personality to the dictates of war. Always focused and disciplined, he allowed nothing to detract from the necessity of defeating Germany. His most famous words may have been "not one step back". The Russian army had been overrun at the start of the German invasion and the civilians were demoralised, hungry and destitute. Zhukov feared that his beloved homeland faced the fate of being a slave state.

He fought with all the ferocity that this deep anxiety provoked. More than 20 million inhabitants of the Soviet Union died in the Second World War. This is a number so huge it almost defies comprehension but it points to the sacrifice made by a nation and to the determination of its leaders to be almost indifferent to human loss. More than 158,000 Russians were also shot by their own side for cowardice and desertion.

"Not one step back" was adopted by Stalin as the rallying cry at Stalingrad. The Russian combatant knew he faced great danger when facing the Germans. He knew, too, that he faced certain death if he turned and ran. This, and so much more, produced a breathtaking brutality. Zhukov has been accused of using weight of numbers to achieve victory and to have been relaxed about the consequent losses. For example, 80,000 Soviet soldiers died in the battle to take Berlin.

However, this ruthlessness was both a product of the times and a necessity in them. He was rude and physically abusive to his officers and to his troops. He was a general who made excellent preparations but was adept at meeting the changing circumstances of battle. But he was no military genius. His greatest characteristic is that he was a general who won battles. This was achieved by careful deployment of resources, backed by a desperation not to let the Nazis prevail.

Roberts is an excellent historian who does not come to conclusions without presenting the evidence and weighing it carefully. This is a brisk, comprehensive biography. But the man escapes capture. The sources on Zhukov all carry some taint of prejudice and his personality only fleetingly appears. It seems Zhukov was a minor philanderer, a major consumer of Russian literature and a man who could be as tender to his loved ones as he could be brutal to those who threatened him.

Roberts has produced a biography that testifies to a substantial body of work, but the heart and soul of Zhukov remain elusive.

Stalin's General: The Life Of Georgy Zhukov

Geoffrey Roberts, Icon, £25