At the time he was living in the flatlands of Schleswig-Holstein, in the far north of Germany, and although his high forehead and angular features betrayed his background, he could just as easily have passed muster as a retired professor.

He was certainly a gentleman and treated my artless teenage queries with due gravity and interest. Yes, he had continued to support Hitler because of his soldier’s oath; but no, he had quickly come to the conclusion that the former corporal had little concept of grand strategy. General Eberhard Mackensen knew what he was talking about and was not attempting any self-aggrandisement: the commander of the 14th Army in Italy in 1944, he had conducted his campaign with aplomb as the defeated German forces fought their way north up the peninsula.

While reading Andrew Roberts’s masterly account of the second global conflict, the memory of that long-lost day came flooding back. Partly it was the book’s subject matter that sparked the reverie, but mainly it was the author’s forceful arguments about the relationship between Hitler and his high command.

Roberts’s thesis is compelling. Hitler was first and foremost a Nazi, and that sense of himself as a man of destiny encouraged him to start the war long before Germany was ready to fight and then to lose it for the very same reasons. In particular, his warped political philosophies turned his anti-Semitic theory into practice, and this blunder was to be Germany’s undoing. Not only did it taint Germans with genocide, one of the foulest of crimes, but it actually harmed the country’s war effort. Military and communications systems were tied down in the monstrous programme to eliminate the Jews of Europe, and the earlier expulsions of the 1930s drove many of the best brains into exile. Lieutenant-General Sir Ian Jacob, Churchill’s military secretary, was not exactly joking when he told the author that the allies won the war “because our German scientists were better than their German scientists”.

But the book’s main dynamic is provided by the sorry outcome of Hitler’s many interferences at the military level. Far from being an inspired amateur who managed to second-guess the generals and beat them at their own game, Roberts’s Hitler emerges as something of a geek, and he says so in so many words: “Because a trainspotter can take down the number of a train in his notebook, it doesn’t mean that he can drive one.”

In fact, the blunders began even before war was declared. If Hitler had postponed the invasion of Poland for a couple of years – there was no strategic need for the assault in the late summer of 1939 – Germany would have been in a much stronger position, tactically and strategically. Instead of going into the naval war with a paltry number of submarines he should have waited until the larger ocean-going boats were developed and he had the necessary numbers to bring Britain to its knees. In 1945 the German navy had 463 U-boats, but by then it was too late. The same is true of the air war. No long-range heavy bombers were produced, and delays in the manufacture of jet-powered fighters and strategic missiles meant they were too few in number and too late in arrival to make much difference.

Even after the war began, Hitler was incapable of keeping his own counsel. In May 1940, as the demoralised British Army made its way to Dunkirk, Hitler ordered von Rundstedt to halt outside the pocket because he was worried his armoured divisions would get trapped in country unsuited to tanks. He also reckoned the soldiers would be exhausted after an intense fortnight and needed to regroup. Charitably, Roberts concedes that Hitler’s reasoning had been fashioned by his own time as a corporal in the trenches during the First World War, but the infamous “halt order” all but ended Germany’s chance to knock Britain out of the war.

From there things got steadily worse. Hitler completely misjudged the situation when German forces invaded the Soviet Union, foolishly promising his people that the Red Army would be knocked out by the summer of 1942. Generals were sacked and then reinstated with bewildering regularity, and Hitler’s response to any problem was the “stand or die” order which was issued at Stalingrad and a few months earlier at El Alamein, the two defeats which presaged the later collapse of Nazi Germany.

In relating all this Roberts does not just dwell on the wider strategic picture. He is good at interweaving personal testimony, so that the massacre of British infantrymen by German SS troops at Wormhout in 1940 is given added poignancy by the story of two sergeants who threw themselves on top of exploding grenades to shield their men.

This being a global conflict, Roberts also turns his attention to the war against the Japanese in the Pacific and southeast Asia, and he has some harsh words to say about Western racist attitudes. Following the easy Japanese victories in Burma and Malaya, he notes that the British had to quickly change their tune about the soldiers who were giving them such a hard time: “From being a bandy-kneed, myopic, oriental midget in Western eyes, the Japanese soldier was suddenly transformed into an invincible, courageous superman.”

On the other hand, Roberts thoroughly approves of the decision to drop the atomic bombs on Hiroshima and Nagasaki as a means of shortening the war, and he is not afraid of saying it was the only medicine which the Japanese high command would accept.

In some respects The Storm of War is a companion to Roberts’s rightly praised Masters and Commanders, which is a wonderfully satisfying exploration of the allied direction of the war. Readers of that earlier volume will have a decided advantage, but this new account possesses many similar virtues. Roberts has the enviable capacity to take on large ideas and make them comprehensible – take, for example, his invigorating examination of the interface between Germany’s invasion of the Soviet Union and the failure of Rommel’s campaign in north Africa. If both had succeeded, it was not beyond the bounds of possibility that Germany could have swept into the Caucasus and the Middle East, thereby grabbing strategic oil supplies and even threatening India. If that had happened, and if Germany had contrived to keep the United States out of the conflict – another of Hitler’s foolish mistakes, according to Roberts – the outcome could have been very different.

All this is related with a good deal of gusto and, as we have come to expect from this scintillating historian, the narrative arrives in a style which manages to be both erudite and entertaining. Once again Roberts shows that he is at the top of his form, combining some hard graft in terms of research with a knowledgeable eye for the variegated scenes and personalities he is so good at delineating.

Although General von Mackensen is long dead, I’m sure that even he would have approved.

The Storm of War is published by Allen Lane, priced £25