There is no single pat answer to the question, “How can one hundred people be led by a single person?” The riddle appeared in the 1981 Cambridge University entrance exam paper and so intrigued the young Andrew Roberts that he has spent much of the succeeding decades analysing its different facets as a multiple prize-winning historian and biographer (Halifax, Salisbury, Churchill, Napoleon). Based on a series of lectures he delivered between 2008 and 2018 to the New-York Historical Society as Lehrman Institute Distinguished Lecturer, his latest book, Leadership in War (Allen Lane), is the distillation of the pressures, character traits and stratagems that nine particularly effectual politicians and commanders deployed.

“The more I see of the full-scale management coursework industry, especially in the United States,” Roberts says, “the more I think business leadership is so different from war leadership that it is a mistake to equate one with the other. It’s well known that great CEOs wind up thinking they are Napoleon. But the requirements and capabilities for leadership in peace, politics and war are so different.”

Roberts’s combination of detailed primary research with stylistic élan, narrative sweep, eye for the telling details and confident, easy familiarity has made him not only a bestselling historian but also a sympathetic and informative confidante to politicians and leaders on both sides of the Atlantic.

Unlike Tony Blair, who he viewed as having no obvious interest in history at all, most of the politicians with whom Roberts has spent time demonstrated an interest in the subject beyond their own place in it. “Margaret Thatcher was fascinated by it and always liked talking about it. Denis loved talking about Lord Alanbrook and the various moments of decision in the Second World War.”

Roberts singles out George W. Bush as a devourer of works of history, belying the caricature created of him as frat-boy ignoramus.

“His reading is very broad, from the American Revolution to the present day. I had dinner with him when I was on one of my book tours earlier this year and he loves to go into the pressures on a decision-maker. He has painted pictures of Churchill so we talked about the various pressures that Churchill was under at different stages in his career. It was an enlightening conversation to talk to someone who has himself been under enormous pressure in wartime. He’s about as different as it’s possible to be from his public image.

Roberts is a Tory historian who blames generations of his brethren for wrongly traducing Napoleon

“I’ve noticed this a lot with various hate figures of the left,” Roberts continues. “Ken Starr is made out to be a puritanical humourless monster. He is in fact an extremely jolly and good-natured chap. The left’s demonology makes Karl Rove out to be a truly sinister figure. He is actually a charming rumbustious good-natured man. I’ve spent some time with Dick Cheney. He is totally different to the person portrayed in the New York Times. And I’m seeing this a bit with Dominic Cummings as well. The idea that he’s like Strelnikov in Dr Zhivago … well, he’s just not like that.

“When you see such people in private life you worry and think, Oh God, maybe that’s true of Rasputin, Lord Bute, or Charles Villiers or all of these figures who over the years have been vilified and continue to be attacked by historians who are relying on contemporary information as they must, but that information of the seventeenth century may be the equivalent of the New York Times or CNN.” He pauses, before adding, “Was Savonarola actually a bit of a laugh?”

Roberts is a Tory historian who blames generations of his brethren for wrongly traducing Napoleon. “It is ridiculous to portray him as a proto-Hitler figure. He opened up the ghettos when his armies came into such cities. He was trying to extend the Enlightenment.”

Roberts’s extensive American audience may be in for a comparable shock to old certainties once his next biography is published, on King George III. “People assume that we lost America because the king was mad. Actually his proper bout of genuine madness didn’t take place until five years after the Treaty of Paris recognised America as an independent country. He is attacked for his involvement with the Stamp Act. But when you look into it carefully, pretty much everything we know about George III is wrong. He certainly wasn’t the tyrant of the Declaration of Independence. In the general psyche, he is thought of as a bad king. His interest in astronomy: Uranus was originally named after him, for example; his interest in agriculture and music, all show otherwise. He was an enlightened monarch, a renaissance prince, who was unlucky to live in the same decades as Benjamin Franklin, John Adams, George Washington and Alexander Hamilton. We didn’t have anything to compare in Grenville, North, Rockingham and Newcastle.”

The Peterhouse, Cambridge, academic, Maurice Cowling, held uncompromisingly to the view that the historian seeking to understand decision-making is better engaged studying the interaction of small, influential, groups than broad socio-economic or cultural trends. It is a top down approach that Roberts endorses. “We didn’t lose America because of what ordinary people thought about turnpikes. We lost it because of what was going on in the House of Lords. An historian ought to look at what is going on in the House of Lords. Unfortunately, this is now looked down upon in the academy.”

But not all the personal interactions of the powerful reward the historian’s time and attention. When asked how important to leadership is having a supportive wife or husband, Roberts can find no common thread. “Denis Thatcher did not have any input into policy although he did ultimately help his wife to stand down. Napoleon took no notice of Josephine or any of his 27 mistresses when it came to decision-making. I don’t think Mamie Eisenhower can be said to have contributed to the broad thrust versus narrow front strategy in 1944-45. Of course it’s better to have a happy home life, and there may be a supportive emotional comfort that should not be discounted. But as far as their drive, decision-making and policies are concerned, I don’t discern in the people I write about any importance to the spouse whatsoever. Look at Harold Macmillan. You’ve got to have a very Edwardian sense of priorities to give a peerage to the person who is sleeping with your wife.”

One of Roberts’s great talents is to convey character through the economy of anecdote. In Leadership in War, he repeats a claim about the Prussian field marshal, Helmuth von Moltke the Elder, who it was said only smiled twice in his life: “The first time when someone suggested that the fortress of Liège was too strong to be captured, and the second time when he was told that his mother-in-law had died.”

History, with its clash of characters, the play of chance and good or poor judgment, the excitement of individuals and of crowds to fight or to flee, courses through Roberts’s scholarship as authentically as through his unabashed enjoyment of life. In such hands, history writing is an exercise not in selectivity and careful tailoring to provide moral lessons or leadership skills that resonate with modern priorities. It is far closer to another reason that has been provided for why we study history — to free ourselves from the tyranny of present-day opinions.