The black & white glass plate photos shown during the credits were taken on set and developed on site by the photographer.
The land used in Minnesota for filming is privately owned by one of the extras. It is a paintball/airsoft field when not being used for movies or law enforcement training. The buildings used in the fight forest were originally built for airsoft and reenacting use.
Just a slight correction to an above comment - WWI ended on November 11, 1918, not 1919.
The helmets worn by the black soldiers are the M1917 Helmet modeled after the British mark 1 Tommy helmets used during the war. However, more commonly black units during the war used the French Adrian helmet. This was due to American leadership passing command of colored units to the French army.
While much of the movie was filmed in Minnesota, the scenes with General Pershing (Ron Perlman) and Colonel Jack Morrison (Billy Zane) were shot in Pasadena, California.