By the first week of March 1915, food supplies inside the besieged fortress of Przemyśl were almost exhausted. Most of the horses that could be spared had been eaten. Bran, sawdust and bone meal were used to eke out the dwindling stock of flour. Cats were nowhere to be seen – they too had been eaten. A middle-sized dog fetched 20 crowns, if its owner could be persuaded to part with it. Even mice were being traded. The hospital was filled to overflowing with collapsing people. As one of the doctors tending them observed, the most shocking thing about the starving was their indifference to their fate. “They silently and without complaint accept a cold place in the hospital, drink the slop which passes here for tea; the next day, they are moved to the morgue.”

One of the marvels of Alexander Watson’s study of the bitter struggle for the fortress in 1914-1915 is his juxtaposition of magisterial technical analysis with scenes of timeless misery. The concrete and steel gun emplacements with their turrets, the fast-firing field artillery and the little wooden aeroplanes that flew in and out of the tiny fortress airstrip were all new, but the stench, the terror, the darkness, hunger and isolation were as old as the art of siege itself.

In the last years before the outbreak of the first world war, Przemyśl , now on the south-eastern edge of Poland near the border with Ukraine, was a high-functioning, multicultural, central European fortress on the eastern marches of the Austro-Hungarian empire. There were attractive cafes and restaurants, theatres and concert halls, lively newspapers catering to a range of ethnicities and political tastes, German, Polish and Ukrainian secondary schools and diverse religions. For all their modernity in technical terms, the two immense concentric rings of fortifications that surrounded the city felt more like reminders of the distant past than warnings of future peril.

Within weeks of the outbreak of war in the summer of 1914, Przemyśl was awash with Habsburg troops making their way towards the Russian armies. When the Russians broke the Habsburg advance, the city filled with the wounded. By 17 September, the Russians had surrounded the city and the first siege began. A renewed Habsburg offensive pushed the front back, briefly relieving the fortress, but the Russians rallied. On 8 October, a second and much more arduous siege began. It would end only when the Austro-Hungarian garrison capitulated to the Russians on 22 March 1915.

Watson lucidly charts the progress of the military campaigns on both sides, deftly situating the struggle for Przemyśl within the larger history of the eastern front. He is sharply critical of the Habsburg command, highlighting their incompetence and the recklessness with which they squandered the lives of their men. Sensible precautions were sacrificed to antiquated ideas of manliness in combat. Cavalry and infantrymen were pushed forward into withering fire. The digging of foxholes was discouraged on the grounds that it “encouraged cowardice”. Towards the end of the second siege, the starving inhabitants of the fortress were harassed by the Habsburg command into mounting an assault on the Russian lines, even though the chance of a breakout was zero. When the order to retreat was finally sounded, the few who had not already been shot dead or wounded were so exhausted that even under heavy enemy fire they could only limp or crawl back to their starting positions. The main villain of the piece is the Austrian chief of the general staff, General Franz Conrad von Hötzendorf, a belligerent martinet who had pressed for war at every opportunity and proceeded to fail at virtually every task he faced. Vain, arrogant, incapable of learning from his mistakes and shockingly insensitive to the suffering of his men, he was the most important single architect of the disasters that befell Przemyśl.

As elsewhere in areas that came under Russian control during the war, the Jews were singled out for special punishment

There is much more to this book than an empathetic and evocative campaign history. Watson uses the fortress city like a jeweller’s glass to show how war distorted and transformed the pre-war civilian world, poisoning it with a spirit of vendetta and ideological hatred that would not swiftly dissipate.

The first non-combatant victims were the Ruthenes, speakers of various Ukrainian dialects who were suspected by the Austro-Hungarian military authorities of favouring the cause of the Russians. On the retreat towards Przemyśl, an infantry regiment of Hungarian Landsturm strung up dozens of Ruthenes from trees at the side of the road. Further atrocities took place inside the fortified rings. As one commander reminisced: “There were some [Ruthenian] villages in which the entire population had to be hanged, because they pulled out Russian rifles … and fired salvos at us.”

Once the Russians were in control, a new wave of repressions began. There were attacks on Ukrainian “Greek Catholic” clergy, whose presence offended the incoming Russian Orthodox authorities. The occupation army embarked on a policy of “Russification”, whose purpose was to restore the region to its supposedly “primordial” Russian condition. And, as elsewhere in the areas that came under Russian control during the war, the Jews were singled out for special punishment. There were repeated waves of arrests, public whippings and forced labour services performed before crowds of impassive Poles and Ruthenians. Sons were made to hang their fathers, before being hanged in turn; 17,000 Jews were simply deported eastward without charges of any kind.

Watson’s handling of these horrors is never gratuitous. In reconstructing in such detail the violence of the war years, he proposes an important rethinking of the history of central Europe in the 20th century. By contrast with Timothy Snyder, whose Bloodlands connects the phenomenon of ideological ultraviolence with the conflict between Stalinist Russia and Nazi Germany in the 1930s and 40s, Watson backdates the inception of ethnic cleansing and racial atrocity to the era of the first world war, when the clashing protagonists were not newfangled totalitarian states, but empires of the old type. Once it was broken, the civility of the old Przemyśl, in which diverse communities, for all their squabbling, had rubbed along tolerably well, did not return.

The power of this book lies in Watson’s ability to always connect such large claims to the experiences of the people who carried this war on their backs. His eye falls on all those trapped in the fortress – the doctors, airmen, merchants, troops entrenched inside the great rings of the fortress, and the sex workers and girlfriends who hoped through love affairs or trysts with officers to lay hands on food they could take to share with their families. And when the siege was over, in March 1915, more than 100,000 troops and civilians of Przemyśl made their way into Russian captivity, most of them fetching up deep in Asian Russia, where around a fifth died. One survivor, who had been 12 at the time of the siege, recalled his father’s return home in 1921: “He came in at the door buckled like a centenarian.”

Watson’s splendid book combines great evocative power (and flashes of sharp humour) with the ethical authority of the best history writing. The story it tells is unsettling, because it resists any attempt to encompass the death and violence of war within a narrative of redemption. It recalls instead a war that never really ended, but rather spilled out into cascades of further violence whose toxic effects are still with us today.

• Christopher Clark’s The Sleepwalkers: How Europe Went to War in 1914 is published by Penguin. The Fortress: The Great Siege of Przemyśl is published by Allen Lane (RRP £25). To order a copy go to guardianbookshop.com. Free UK p&p over £15.