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, although in the movie, filming took place in Utah, as a stand in for Montana.
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, named for Princess Patricia of Connaught, originated in Ottawa, Ontario, Canada in August 1914, later based in the area of Edmonton, Alberta, Canada, and which continues as an active unit as of 2015. The "Patricia's" celebrated their 100th anniversary in 2014.
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, so the film's Frederick is clean lipped. Just one year later, Holden did agree to wear a mustache (a false one) in The Wild Bunch (1969) after several arguments with Sam Peckinpah.
The two actors who play the senior Canadian officers had both previously played future Presidents of the United States in military service. Harry Carey Jr. played Dwight D. Eisenhower as a West Point cadet in The Long Gray Line (1955), and Cliff Robertson played John F. Kennedy as a World War II Navy lieutenant in PT 109 (1963).
On February 5 2015, surviving members of the "Black Devils" upon which this movie was based--all in their 90s--received the Congressional Gold Medal, the highest civilian award, for their service in WWII.
This movie has some of Utah's most famous boxers. Gene Fullmer who was a world Middleweight champion famous for beating Sugar Ray Robinson twice. Gene's brother Don who was an American Middleweight Champion, and Rex Layne who once challenged Rocky Marciano for the World Heavyweight Championship.