No one knows how many died, but the Russians officially claim they lost some 20 million lives – though do not say how many died in battle, how many civilians were bombed, murdered or starved to death and how many soldiers were summarily shot by their own side as deserters or defeatists. What is clear is that without the Red Army, Hitler would not have been defeated. And 60 years after victory, the Russians still regard the 'Great Patriotic War' as their most heroic achievement, and one that saved the world from Nazi domination. So titanic was the struggle, however, that the details have been lost – or deliberately concealed. No proper record exists of what life was really like on the front line. What did the average soldier, usually a villager who had never gone beyond his collective farm, think? How did he survive? What motivated him? How did an army that lost more men in three months than Britain in both world wars recover from disastrous incompetence, mismanagement, lack of weapons, training, food and morale to smash the Wehrmacht and capture Berlin?

In Ivan's War, Catherine Merridale has delved deeper, more lucidly and comprehensively than anyone before to find answers. It is a monumental achievement: not only has she read thousands of letters, secret reports, long-closed NKVD archives, military communiques and volumes of official history, but she has searched out some 200 veterans, men and women, and persuaded them to set aside the propaganda, dredge up the horrors they witnessed and pass on the fears, panic, atrocities, songs, hopes, obscenities, Stalinist persecution, chaotic orders by ignorant political placemen, bestial conditions and bestial cruelty. It was not easy: memories have faded, few wanted to recall what it was once politically dangerous even to think and most would not tell their stories to a woman. But, with male Russian helpers, she teased out of them a compelling record.

As she says, "This war defied the human sense of scale": by December 1941, six months into the war, the Red Army had lost 4.5 million men. One single campaign, the defence of Kiev, had cost the Soviets nearly 700,000 men killed or missing in a matter of weeks. From 1941 until VE-Day, about 30 million men had been mobilised and the Red Army was destroyed and renewed at least twice, with casualties running at roughly 14 times the rate of the tsarist army in the First World War.

Communist ideology was largely responsible for the initial disasters. Propaganda insisted that communism would swiftly overwhelm the capitalists, painted an utterly false picture of the outside world and refused to allow individual initiative or accept the realities of German strength and the earthy realism of the few experienced Soviet soldiers. Morale was rock-bottom: not only had Stalin's purges decapitated the Army leadership, but the carnage of the Finnish 'winter war' in 1939 had taken a huge toll on lives. Then, as in 1941, the chaos was indescribable. Stinking, lice-infested troops had nowhere to sleep, inedible food, few weapons (they were not even trusted with proper guns), no hospitals, maps or battle plans and above all, no leadership. The 'politruks', the despised but deadly political spies and propagandists, obstructed everything, countermanded military orders and sabotaged all initiative.

Contrary to subsequent myth, Stalin was not widely loved or respected, peasants had bitter hatred for communism and restless national minorities went over to the Germans. Even at the height of battle, when the tide was turning after Stalingrad and with NKVD troops at the rear to fire on anyone retreating, men deserted in droves. Just before the battle of Kursk, some 6,000 troops deserted. Some 158,000 men were formally sentenced to be executed during the war, and at least 422,700 were sentenced to serve in penal battalions, ordered forward to almost certain death. It was not just the unbearable conditions, the hunger and the panic: there was also desperate worry about families lost or under German occupation. And yet, despite all this, the Red Army prevailed. Merridale dwells on what kept it going: the deep Russian patriotism, genuine horror at German atrocities, comradeship – even if it lasted only weeks before death intervened – fast-learnt leadership, better training and the gradual replacement of political appointees by tough, seasoned officers. Stalin finally listened to his generals. The massive war effort paid off: by 1943, the Soviets were turning out T-34 tanks, the mainstay of the war, at a rate of over 1,200 a month. The technology was simple but effective, the tactics became sophisticated and morale rose as propaganda fanned tales of heroism.

The books covers every aspect of daily life: sex (very little of it), religion (tenaciously present), politics (omnipresent) and the powerful thirst for revenge. Merridale seamlessly weaves the anecdotes with the narrative, the war history with the battlefield experience of individual soldiers, the official claims and the grim reality. She follows the Red Army as it fights back through ruined villages, past the hideous liberated concentration camps and into the ruins of Berlin. It was an eventual triumph. But at a stupendous and unimaginable national and human cost.