A "daringly politically incorrect" book written by a little-known Canadian academic last night defeated the acclaimed master of modern political biography, Roy Jenkins.

Margaret MacMillan became the first woman to win the £30,000 Samuel Johnson prize, the UK's biggest non-fiction award, with her study Peacemakers: the Paris Conference of 1919.

The result was understood to be a split decision by the judges.

In the book, Ms Macmillan, a professor of history at Ryerson University, Toronto, contests more than 80 years of conventional wisdom by arguing that the 1919 international conference - which included the Treaty of Versailles - cannot be blamed for causing the war against Hitler 20 years later.

One leading reviewer called her stand "splendidly revisionist and daringly politically incorrect".

The former politician Roy Jenkins was the initial runaway favourite to win the prize. His bestselling Churchill, a study of the second world war leader, has been widely praised as the best book on politics and statesmanship for some years. It is regarded as almost the equal of his earlier biographies of Gladstone and Asquith.

According to the bookmakers William Hill, Jenkins' "truly magisterial" study was 2-1 favourite to win the Johnson award. Second at 3-1 was William Fiennes's The Snow Geese. Peacemakers was quoted at only 6-1, last of the six shortlisted works. But it attracted a late surge of bets after winning good reviews.

Announcing Ms MacMillan's triumph live on BBC 4, the chairman of the judges, the broadcaster David Dimbleby, said: "The judges felt they had a shortlist of great quality and diversity: a major work of modern history, of biography, of science, of Tudor history, of travel writing and of contemporary polemic.

"After a vigorous debate, our choice was Peacemakers by Margaret MacMillan, a book which challenges the conventional view of the Versailles conference, whilst bringing vividly to life an extraordinary event which shaped the 20th century and still resonates today."

A factor thought to have counted against Roy Jenkins in the final stages of the contest is his confession that, in writing Churchill, he carried out no original research and relied on previously published material.

One commentator said its shortlist contained "six titles of real distinction, collectively more interesting than any six novels published in London last year".

The other three titles were Eamon Duffy's The Voices of Morebath, a study of religious change in a mediaeval Devon village - which was regarded as a strong contender for the prize; The Invention of Clouds, by Richard Hamblyn; and Brendan Simms's Unfinest Hour: Britain and the Destruction of Bosnia.

The Johnson prize celebrates all non-fiction, from biography and popular science to sport and is in its fourth year.

The BBC agreed to sponsor the prize earlier this year in a three year deal. The award ceremony will be shown on the new arts and culture digital channel BBC4.

Extract from Peacemakers: the Paris Conference of 1919

Later it became commonplace to blame everything that went wrong in the 1920s and 1930s on the peacemakers and the settlements they made in Paris in 1919... Eighty years later the old charges still have a wide circulation.

"The final crime," declared the Economist in its special millennium issue, "was the Treaty of Versailles, whose harsh terms would ensure a second war."

The peacemakers of 1919 made mistakes, of course. They took pains over the borders of Europe, even if they did not draw them to everyone's satisfaction...
If they could have done better, they could certainly have done much worse.
They could not forsee the future and they certainly could not control it. This was up to their successors.

When war came in 1939, it was a result of 20 years of decisions taken or not taken, not of arrangements made in 1919.