My impression has been that historians have generally treated King as the wannabe in this group, a sort of headwaiter at occasional meetings between the British and American leaders. Thompson makes a case that such is a serious misreading.
King knew both Roosevelt and Churchill much better and longer than either of them knew the other. In Thompson's telling, King had stronger relationships and more influence with both of them than has been thought. He gives substantial evidence for the view that King significantly facilitated and sometimes influenced the vital Roosevelt-Churchill relationship, not least by usefully interpreting each to the other on several occasions.
Thompson is impressively well-informed about British, American, European, and Canadian politics in the mid-twentieth century and can constantly flesh out minor details, drawing on seemingly every memoir and biography of the period ever published. A key source for him, however, is Mackenzie King's own diaries, which he employs constantly to cover King's encounters with the other leaders back to 1900. King's notes enable him to flesh out what British and American scholars have often covered in regard to Roosevelt and/or Churchill without drawing on King's diary.
It's an impressive and readable book on what might seem a well-trodden topic, and Thompson's confident familiarity with the period and its people makes for engaging reading. It's marred, unfortunately, by way too many small failures in copyediting and proofreading. Sutherland House, a welcome new venture in Canadian publishing, may need to up its game in that regard.
The underestimation of Mackenzie King, not exclusively by non-Canadians, is also demonstrated in a recent review by the Guardian film critic Peter Bradshaw of a Canadian film, "The Twentieth Century" by Matthew Rankin. If Bradshaw is accurate, the film "satirises the country’s stolid colonial traditions" and portrays King as "subservient to the British colonial establishment" and "a properly pompous and pointless Canadian prime minister." Well, it's only a movie; directors can say what they want.
But if you are at all susceptible to this kind of self-hating Canadianism, Thompson's book could be a useful corrective. One of The Third Man's themes, in fact, is King's endless, skillful, and effective campaign to keep Canadian wartime policy as free as possible from Churchill's imperial aggrandizing and Roosevelt's new American empire. King, indeed, was one of the reasons Churchill failed to keep his promise not to preside over the breakup of the British Empire.
Active History recently posted Sean Graham's interview with Neville Thompson about The Third Man.