A few hours after Neville Chamberlain resigned as the British prime minister, Winston Churchill, who had conducted a lonely struggle throughout the 1930s against appeasing the Nazis, began to assemble his Cabinet. “I was conscious of a profound sense of relief,” he recalled. “At last I had the authority to give direction over the whole scene. I felt as if I were walking with destiny, and that all my past life had been but a preparation for this hour and for this trial.”

Certain he would not fail, Churchill went to bed at 3 a.m. and slept soundly. When he woke up, he said to his wife, “There is only one man who can turn me out and that is Hitler.”

In “Churchill: Walking With Destiny,” Andrew Roberts, the author of “Storm of War: A New History of the Second World War” and “Napoleon: A Life,” draws on Churchill’s letters, speeches, transcripts of war cabinet meetings, and the recently released diaries of King George VI and Ivan Maisky, the Soviet ambassador to Great Britain, to provide a monumental and masterful biography of one of the most important political leaders of the 20th century.

Mr. Roberts takes seriously Churchill’s admonition that “discriminating criticism is necessary” to “do justice to a great man.” And “gush, however quenching, is always insipid.” He finds fault with Churchill’s judgment on a wide range of issues, including his opposition to woman’s suffrage, continuing the Gallipoli invasion during World War I, supporting Edward VIII during the abdication crisis, and remaining as prime minister after his stroke in 1953.

Mr. Roberts also questions Churchill’s skill as a military strategist. He acknowledges that he was stubborn, selfish, intolerant, overbearing and racist.

That said, Mr. Roberts demonstrates that Churchill learned from his mistakes, exhibited extraordinary physical and moral courage and was prescient and profound in warning that Nazism and Communism posed existential threats to democracy. By turns confident, combative, emotional and eloquent, Churchill was an exemplary war leader.

Andrew Roberts delivers the exhaustively researched, comprehensive and balanced biography Churchill clearly deserves. His narrative, however, occasionally bogs down under the weight of unnecessary details. Mr. Roberts informs readers of every Cabinet shuffle. He tells us that the prime minister placed Splash, the stuffed duck-billed platypus given to him by Australia’s foreign minister, in the lobby of No. 10 Downing Street, and regaled visitors with information about its love life.

Mr. Roberts repeats — and repeats — his claim that Churchill “enjoyed alcohol but almost never excessively.” At times, he ends paragraphs by belaboring the obvious, declaring, for example, that attacks on family and friends “brought out the tiger” in Churchill.

Happily, the vast majority of the details in the book illuminate the extraordinary character of this extraordinary man. While Hitler never visited a bomb site, “driving past such scenes with the curtain of his Mercedes-Benz drawn,” Mr. Roberts writes, Churchill frequently visited the East End during the Blitz to boost morale. He also closely monitored rationing and lobbied the Commons to increase compensation for homes destroyed or damaged by Nazi bombers.

In peace and war, at gambling tables, in the stock market and while driving his car, Churchill was a “compulsive risk-taker,” Mr. Roberts reveals. Although this tendency led to mistakes, it contributed, mightily as well to victory.

Churchill’s former girlfriend, Pamela Plowden (later the Countess of Lytton), Mr. Roberts suggests, got it right when she opined “The first time you meet Winston you see all his faults, and the rest of your life you spend in discovering his virtues.”

Thanks to Andrew Roberts, we can see both, and then celebrate him as a flawed genius, master of the English language, passionate but cool under fire, who at age 65, when almost everyone dismissed him as a has-been, seized the moment, and helped save Liberty and Democracy.

Glenn C. Altschuler is the Thomas and Dorothy Litwin Professor of American Studies at Cornell University.

First Published: December 11, 2018, 12:30 p.m.