A fair point Zoidburg, I was thinking of this passage from Naples '44 by Norman Lewis, a Field Security Sergeant in North Africa and Italy in WW 2.
'September 28th.
Admitted to the American 16th Evacuation hospital at Paestum with malaria......Most of the patients have battle wounds, and from several of these I have received confirmation of the story I found so hard to believe, that American combat units were ordered by their officers to beat to death Germans who attempted to surrender to them. These men seem very naive and childlike, but some of them are beginning to question the morality of this order. One man who surrendered to a German tank crew was simply stripped of his weapons and turned loose because he could not be carried in the tank, and as a result he is naturally a propagandist for what he accepts as the general high standard of German humanity.'
The Rangers would be a special case, but Saving Private Ryan brings the issue into the open more than is usually the case in US films.
The effect of the reputation that the US gained was to produce series of reciprocal atrocities, Malmedy was one committed by the SS.
http://www.30thinfantry.org/malmedy.shtml Here is an extract from an account of that incident.
'Between 1500 and 1600 hours vehicles of Kampfgruppe Peiper fired into the bodies lying in the field as they passed through the crossroads. But amazingly there were survivors of the massacre. Of the original members of the 1st serial of B Company a total of 55 men survived the ordeal. Some of the soldiers escaped during the initial attack and some escaped after they were captured. Some of them were recaptured by other German units, (they did not say anything about the shootings at Baugnez until after the war for fear they would be shot for being a witness to a war crime), and some made it to friendly lines to tell of the shootings. The last one took four days to make it to friendly lines.
When these men told their tale of the shootings at Baugnez, it enraged the Americans and inspired them to fight with conviction and with little compassion towards the enemy, especially towards SS or Fallschirmjäger soldiers. In fact, there has never been found a written order by a German commander to kill American prisoners, but there were orders written by American regimental commanders that directed that all SS and Fallschirmjäger would be shot on sight.'
Damon.