According to the Australian DVD sleeve notes, "The U.S. Department of Defense provided 300 members of the Utah National Guard to play soldiers in the mass battle scenes filmed on Wasatch Mountain."
Robert T. Frederick, the commander of the Devil's Brigade, had a mustache in real life, but William Holden, who reportedly did not like his own image on film with a mustache, refused to grow or wear a false one while portraying Frederick in the movie.
The Devil's Brigade as seen in this movie was based on the true story of the United States First Special Service Force.
The Devil's Brigrade actually existed (although the unit was actually known as "The Black Devils"). During World War II, the brigade suffered casualty rates of 39%. Following the end of WWII, the brigade was disbanded. Veterans of the Devil's Brigade have been meeting each year, since 1945, in Montana, at the former training facility depicted in the movie.
When O'Neill arrives his regiment patch on his shoulder reads PPCLI. This stands for the Princess Patricia's Canadian Light Infantry, a real regiment based in Alberta that is still active today.