There is a good book to be written about the myriad of moral dilemmas facing individuals and institutions during World War II. You are the mayor of a small town where the Nazis are about to shoot 50 innocent civilians if you do not disclose the whereabouts of a wounded resistance fighter who you think might be about to die anyhow. You are the captain of a unit ordered to hold out against the SS until dusk, but you’re surrounded and out of ammunition and you are threatened with massacre if you don’t surrender before 5:30 p.m. You are the sergeant ordered to use bayonets if necessary to force Cossack women and children onto cattle-trucks to be taken back to certain death at the hands of the partisans. What do you do?

Sadly, Michael Bess’s “Choices Under Fire: Moral Dimensions of World War II” (Knopf, 416 pages, $27.50) is not such a book. Instead, it is yet another indictment of the British and American high commands for committing war crimes‚ such as the indiscriminate butchery of the undeniably atrocious‚ carpet-bombing of Germany, and the destruction of Hiroshima and Nagasaki. Posing as an author who explodes myths, the Vanderbilt University history professor merely reinforces them, adding a few morally disgraceful sentences of his own, such as this one about V-E Day: “Badness was actually having a good day on May 8, 1945.”

For Mr. Bess, the attack on Pearl Harbor grew out of Japan’s searing experience of helplessness before European and American domination in which Europeans and Americans had figured very prominently as aggressors. According to him, the judgments handed down to the Nazis at Nuremberg represented rough victors’ justice, rather than morally clean verdicts. (Of course, it ought to have been rougher, since nice, apologetic, articulate, middle-class Albert Speer escaped the noose while his rougher-hewn lieutenant Fritz Sauckel swung for precisely the same crimes.)

Mr. Bess admits that he views the morality of World War II through the prism of an anti-Vietnam War documentary that deeply affected him as a senior in high school in the mid-1970s, and therein lies the problem with this book. He writes about the fearfully difficult decisions of the 1940s from the smug position of the 1970s. It is thus utterly anachronistic to state, as Mr. Bess does, that because the Allies could be fairly certain by November 1944 that Hitler was not likely to deploy a nuclear bomb before he was defeated in May 1945,that the most urgent rationale for developing atomic weapons faded away. Allied leaders were morally at fault, according to this analysis, for continuing to press for a weapon they would not need.

Yet in November 1944 the Germans were by no means defeated. The very next month Hitler staged the stunning Ardennes counteroffensive which led to the Battle of the Bulge. No one knew for certain whether the Führer’s threat of secret terror weapons‚ was true. There was fear of a Bavarian defensive redoubt and V-2 rockets were still falling on London as late as March 28, 1945. To put the Allied nuclear project on hold because of something that was not considered likely would have been an act of criminal negligence by the Anglo-American decision-makers, even if we ignore the vital necessity of atomic weaponry in winning the war against Japan.

Similarly, Mr. Bess’s strictures against the Roosevelt administration for interning Japanese-Americans in 1942 needs to be seen in its proper historical context. Although 64% of the 110,000 Japanese interned were American citizens, that still leaves 36% — or 40,000 people — who were not. With the level of danger posed by Imperial Japan in the spring of 1942, no country on earth would have allowed so many non-citizens of the same ethnic background as the prospective invader to reside in Hawaii and California, where the next blows were expected to fall. Instead of leading sacred cows to the abattoir, as advertised, all that Mr. Bess’s book does is nurture them as they chew their cud.

By total contrast, Hugh Sebag-Montefiore’s book “Dunkirk: Fight to the Last Man” (Harvard University Press, 720 pages, $35) is well-judged and fascinating, providing the right background for the disastrous campaign. Despite its initially confusing subtitle (the whole point of Dunkirk was that the British and French didn’t have to fight to the last man), this book provides convincing answers to all the important questions raised by the Allied debacle.

Mr. Sebag-Montefiore gives a narrative account of the whole period from May 10, 1940, a fortnight before the evacuation from Dunkirk began, until June 18, 1940, nearly a fortnight after it had ended. Although his central story is of course the withdrawal of the British Expeditionary Force and part of the French Army from the Dunkirk beaches, he also covers many important aspects of the story incidental to that. It soon turns out that his subtitle refers not to the 338,000 men who successfully escaped from the continent, but to those heroic British and French units that held the perimeter for long enough for the rest of the force to cross the English Channel. Without their sacrifice, the BEF would have been captured and World War II effectively lost.

There is still much contested about the retreat to Dunkirk and its aftermath. Was it strictly necessary for Winston Churchill to sacrifice the 51st Highland Division at St. Valery? Should a second expeditionary force, under General Sir Alan Brooke, have been sent to Cherbourg? Did the Belgian fifth column assist the Nazis? Should more Royal Air Force fighter squadrons have been expended in the Battle for France? Did the King of Belgium capitulate too soon? Were the massacres of British Prisoners of War properly prosecuted after the war? How many people drowned on the troop ship Lancastria, and was there a cover-up? Above all: Whose fault was it that the Wehrmacht sliced through the Allied armies, just like the sickle after which their invasion strategy was code-named? Mr. Sebag-Montefiore provides sane and convincing answers in each case, while still giving the reader plenty of room to dissent.

In the chaos and horror of May and June 1940, there are, of course, any number of compelling human stories. While explaining the overall strategy — helped by 30 pages of excellent maps — Mr. Sebag-Montefiore has expertly interwoven individual tales of heroism and occasional cowardice. For all the inspiring Victoria Cross-worthy stories of men like Sergeant-Major Augustus Jennings or Lieutenant Dickie Furness, who sacrificed their lives for their comrades, there were others who tried to rush the mole at Dunkirk in order to get home safely.

While a mixed party of men was forming up to embark, the naval lieutenant Sam Lombard-Hobson recalled,

a single soldier, unable to take any more, broke ranks and made a dash for the gangway. Without a moment’s hesitation, the subaltern in charge took out his revolver and shot the man through the heart, who lay motionless on the jetty. The young officer then turned to his section, and calmly told them that he wanted only fighting men with him. The effect was electric, and undoubtedly prevented a stampede by other troops awaiting evacuation.

The fact that Britain had any fighting men left after the debacle on the Continent was largely due to the miracle of Dunkirk. Several fine books have been written about it, but none better than this. Perhaps it should be Mr. Sebag-Montefiore who one day pens the important book on the morality of 1939–45 warfare that Mr. Bess has unfortunately failed to write.

Mr. Roberts’s “History of the English-Speaking Peoples Since 1900” will be published in February by HarperCollins.