Patrick J. Buchanan's Churchill, Hitler, and the "Unnecessary War" gets part of its title from a quote by Winston Churchill. Buchanan has brought the term up in several interviews a statement made by Winston Churchill.

In his memoirs, Churchill, who led Britain to victory in World War II, wrote:

One day President Roosevelt told me that he was asking publicly for suggestions about what the war should be called. I said at once, "The Unnecessary War." There never was a war more easy to stop than that which has just wrecked what was left of the world from the previous struggle.Patrick J. Buchanan, Churchill, Hitler, and "The Unnecessary War" (New York: Crown Publishing Group, 2008), xviii.

Buchanan is quoting Churchill's preface to The Gathering Storm, the first part of a 6-volume series about World War II.Winston Churchill, The Second World War: The Gathering Storm (Boston: Houghton Mifflin Company, 1948), iv. When bringing up this quote, Buchanan typically follows it up with reversing the term back on Churchill.

The war was unnecessary, Churchill said, because of the constant blunders before the war that got us into it. It was the easiest war to avoid in all of history. That's what Churchill told Franklin Roosevelt and he was right. What he didn't say was a number of those blunders had been committed by Winston Churchill himself. Interview with Stephen Colbert, June 2008. video

What were the blunders that Churchill was referring?

After the preface to The Gathering Storm, Churchill states that the theme of the book is "How the English-speaking peoples through their unwisdom, carelessness, and good nature allowed the wicked to rearm"Winston Churchill, The Second World War: The Gathering Storm (Boston: Houghton Mifflin Company, 1948), ix..

The book is a narrative of events after World War I to the point of Churchill becoming Prime Minister in 1940. Churchill himself best sums up the reason for labeling World War II the "Unnecessary War".

In a speech Churchill gave as he was working on he first chapters of the book.Martin Gilbert, Churchill: A Life (New York: Henry Holt and Company, 1991), 862. Churchill brought up the anecdote about his conversation with Roosevelt while giving a speech to the Belgian Senate and Chamber. On November 16, 1945, Churchill stated:

President Roosevelt one day asked what this War should be called. My answer was, "The Unnecessary War." If the United Stated States had taken an active part in the League of Nations, and if the League of Nations had been prepared to use concerted force, even had it only been European force, to prevent the re-armament of Germany, there was no need for further serious bloodshed. If the Allies had resisted Hitler strongly in his early stages, even up to his seizure of the Rhineland in 1936, he would have been forced to recoil, and a chance would have been given to the sane elements in German life, which were very powerful especially in the High Command, to free Germany of the maniacal Government and system into the grip of which she was falling.

Do not forget that twice the German people, by a majority, voted against Hitler, but the Allies and the League of Nations acted with such feebleness and lack of clairvoyance, that each of Hitler's encroachments became a triumph for him over all moderate and restraining forces until, finally, we resigned ourselves without further protest to the vast process of German re-armament and war preparation which ended in a renewed outbreak of destructive war. Let us profit at least by this terrible lesson. In vain did I attempt to teach it before the war.Robert Rhodes James, ed. Winston S. Churchill: His Complete Speeches, 1897-1963. Vol. 7, 1943-1949 (New York: Chelsea House Publishers, 1974), 7251.