On April 12, 1933, four prisoners were gunned down in scrubland at a detention centre in the town of Dachau, on the outskirts of Munich.

Prison guards said they opened fire after Arthur Kahn, a 21-year-old medical student due to enrol at Edinburgh University, tried to escape by sprinting for a thicket that ringed the one-time gunpowder factory. Three more inmates also bolted. The victims had one thing in common: all were Jewish. The men who killed them belonged to the SS, the Nazis' infamous security corps.

The premise of historian and journalist Timothy W Ryback's forensic, unflinching and utterly compelling new book is that these cold-blooded murders, the first at Dachau, presaged the death of millions. The killings "involved the constituent parts of the genocidal process - intentionality, chain of command, selection, execution - we have come to know as the Holocaust."

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The Dachau shootings did not go unnoticed, even in the febrile atmosphere of 1933 Germany. A courageous public prosecutor, 39-year-old Catholic army veteran Josef Hartinger, vowed to bring the culprits to justice. Pieced together from Hartinger's contemporary records and later reflections, Hitler's First Victims is as much about the frustrated efforts of Hartinger, the archetypal "Good German", as the routine torture and summary execution that quickly became the norm at Dachau, and then at camps across Europe.

By 1933, Germany was a nation teetering on the brink of totalitarianism. Hitler used the conflagration that engulfed the Reichstag that February to consolidate his grip on power. Anti-Semitism was accepted, even encouraged. (One of the quartet of victims at Dachau, left-wing activist Rudolf Benario, was earlier expelled from the student council at his Erlangen university for the crime of being Jewish.) Internment without trial became the norm for opponents of the Nazis.

The 530 initial detainees housed at a former munitions works at Dachau mostly comprised Communist leaders, but there were also doctors, lawyers, and even two members of the Bavarian state parliament.

Today, Dachau is a name redolent of barbarity and the worst in mankind but in 1930s Germany it was renowned for its beauty, its cobblestone streets and cross-timber façades popular with artists and writers. Dachau was also iconic for German Communists. The short-lived Bavarian Soviet Republic won a fabled battle here in 1919.

The Soviet Republic quickly perished in an orgy of violence, as right-wing "Freikorps" routed the Communists in Bavaria. In early 1933, SS commander Heinrich Himmler handpicked a Freikorps veteran, Hilmar Wackerle, to oversee the newly opened Dachau camp. A former farm manager, Wackerle "knew something about keeping living stock in barbed-wire enclosures". His favourite pastime was "Judensport". He liked to flail Jewish prisoners with a dried ox pizzle. Deaths became a regular occurrence.

Exemplars of what Hannah Arendt later called "the banality of evil" abound. One Jewish prisoner at Dachau was beaten so badly that he grew delirious. He was tied to his bed, salt poured into his gaping wounds. Overnight he died. Next morning his corpse thrown into a shed and subsequently set alight. No one was charged with the crime.

The Nazis' success, as Ryback shows time and again, was predicated on replacing state structures with the party's own apparatuses. The SS assumed responsibility for running detention centres; Himmler himself became police chief. The state bureaucracy was filled with Nazi sympathisers. An alternative "Foreign Political Office" was even created to circumvent the established foreign ministry when diplomats in German embassies abroad refused to fly the Swastika, citing international protocols.

The international failure to halt Hitler's advance has been well documented but demands constant retelling. On the morning of April 12, as the four prisoners that would later be shot at Dachau were being flogged, Nazi emissary Papen was sitting down in the Vatican with Pope Pius XI. Just weeks later, US president Roosevelt celebrated the "excellent impression" made by visiting Nazis. Journalists, too, failed to convey the growing horror of the camps to the outside world.

There are discomfiting echoes in the present day. President Hindenburg enabled Hitler in part because of his anger at what he perceived as Weimar's lax immigration policies. Right-wing parties across Europe espouse the same message. Recently, anti-Muslim demonstrations in Dresden have attracted tens of thousands. German authorities have unveiled plans to house 21 asylum seekers in Buchenwald.

In 1933, Josef Hartinger failed to bring the Dachau killers to justice, but his efforts were not in vain. Twelve years later, at Nuremburg, the Bavarian prosecutor's notes provided the backbone for the legal case that the SS were a criminal organization with homicidal intent from its infancy.

Ryback's prose is well paced and highly readable, his conclusion unerring. Dehumanisation - whether of Jews, Muslims, Communists, "infidels" or anyone else - is often the first step on a blood-stained road. The four shootings at Dachau in April 1933 "were repeated a millionfold with the mobile killing units that followed the path of the German armies across Eastern Europe".

As the 70th anniversary of the liberation of Auschwitz approaches and storm clouds gather over Europe again, the story of the first killings at Dachau has scarcely been more urgent.