Farley goes on the state: "Perhaps unsurprisingly, the characteristics that make an army likely to commit atrocities also make it ineffective on the battlefield."
Maybe in Asia, but not so much in the European war. The German army of 1940-43 was hardly "ineffective" and, by late 1944, the Red Army had finally become a modern and irresistible force (thanks in part to an infusion of American weaponry and transport). The Red Army's notorious rapes and other atrocities in East Prussia and later, during the siege of Berlin, occurred at the peak of its military efficiency.
The difference, of course, is that the advancing Red Army was officially encouraged to seek revenge for years of atrocities perpetrated on fellow soldiers and citizens by the Germans in the Soviet Union. Catherine Merridale, in her fine book Ivan's War (2006), estimates that 7.5 million civilians died during the Nazi occupation of Russia, Ukraine, Belorussia and other areas of the Soviet Union between 1941-45. Altogether 8 million Soviet soldiers died, including more than 3 million Soviet POW's who were shot, starved, worked to death or died of disease or medical experiments in conditions so vile that they can scarcely be imagined. Only 900,000 Soviet POW's were still alive in German prison camps when the war ended, for a mortality rate of 73%. (Of 3.1 million Germans sent to Soviet POW camps, about 474,000 or 15% died in captivity.)
The desire to avenge these deaths, and the widespread destruction of Soviet towns and cities, was a powerful motivation for the Red Army as it campaigned through East Prussia and Germany, pillaging on a large scale. Rape was common despite official condemnations (and even some executions), though Soviet archives contain little reliable data on its scale.
By contrast, revenge was less of a motive for atrocities by Japanese troops when U.S. marines landed on Iwo. The firebombing of Japanese cities by American B-29's had barely begun by February 19, 1945, when the 35-day battle for Iwo began (1). Since the beginning of the conflict in the Pacific, summary execution of those attempting to surrender was routine and expected by both sides.
Under the code of bushido, it was a sign of moral weakness and disloyalty for anyone, including enemy soldiers and civilians, to submit to capture. Prisoners, therefore, lost all claim to respect and humane treatment in the eyes of their captors (2). The bushido tradition explains both the mass suicides of Japanese soldiers (and, on Saipan, civilians) and the atrocities inflicted on their military and civilian captives.
As for "Letters from Iwo Jima," I thought it was historically accurate and exceptionally well done, especially the acting and cinematography. It was a bloody but ultimately pointless battle fought by Japanese and Americans whose lives were grossly undervalued by their military leaders. Both Eastwood films present Iwo from their point of view as a kind of social history of a battle whose intensity and carnage was exceeded only by the U.S. invasion of Okinawa exactly 62 years ago today.
(1) The cataclysmic raid on Tokyo occurred on March 9-10, 1945, killing an estimated 72,489 civilians in what was likely "the most devastating single raid ever carried out by aircraft in any war including the atomic bombings of Hiroshima and Nagasaki, and the firebombing of Dresden." The battle for Iwo Jima ended about two weeks later.
(2) The death rate for allied POW's in Japanese camps was about 37%, slightly higher than the death rate for German POW's in Soviet camps. By contrast, the rate for British and American POW's in German custody was 3.5% from 1939-45.