When war was declared in 1914, Colonel Leonard Messel was unable to serve overseas because of his German ancestry. So he devoted his time to the UK training battalions of the Royal East Kent Regiment, and formed close, long-lasting bonds with the men.
Col Messel, who lived at Nymans, West Sussex, kept a bound book of the letters, which he treasured, and now selected contents, along with audio recordings, are to be displayed at the house - now run by the National Trust - for the first time.
A Stanley Peters, 2/Lieut, wrote from a hospital in Rouen, France, on August 11, 1917: "You will guess by the address that I have been hit again but it's not very much.
"The same night poor Sherren was killed instantaneously. We all miss him - he was splendid in the line.
"Far worse than the shelling, the machine gun and this new gas (which leaves big blisters) was the mud. It was indescribable. Several of my chaps went in up to their armpits and I am afraid quite a number never got out."