Title says it all. It’s a classic war movie, but I can’t remember if it was color or black and white. Maybe the main characters are British. Anyway, a spy gets the bright idea to make a nurse write a love letter to her sweetheart on the front, intending for it to be intercepted by the bad guys. Of course they do intercept it, and get the bright idea that they’re going to blow up their water wells in their main city to prevent the good guys from using the city. In the end, they are prevented from doing so, and the day is saved.
“The Lighthorsemen” (1987)
Detailed plot on Wikipedia here:
I am sure that is the movie you are looking for. If you agree, then please select my answer as Best Answer, there is an option for that.