Corrected entry: The entire premise of the movie is flawed. If the General needed to get an order to Col McKenzie, the quickest and surest means would have been to drop a message from an aircraft. In fact, the field beyond the aid station at the end of the movie would have been perfectly suitable for an aircraft of the time to land and hand-deliver a message the day before the assault.
Ssiscool
25th Jun 2020
Correction: The field beyond the aid station at the end of the movie was also in a prime position to be bombarded by enemy artillery. Regardless, this is not how messages were sent during WWI.
The military used people called runners especially for this purpose of delivering urgent messages. Co-incidentally, Adolf Hitler was a German runner in WW1 who was injured in 1916.
Ssiscool ★