While discussing Peters letter to the Committee, and looking at a map with my parents this afternoon, I noticed a town called Chateau Thierry on the road between Paris and Reims. As Cherry and Thierry sounds almost the same, my guess is Peter either didn't know or couldn't remember how to spell Thierry. Further, the river Marne runs through the area, and there was a huge battle in the area during the war.
When I got back home, I looked in my books about World War 1, and in a Danish book found the following text:
"Another two offensives followed, and suddenly the Germans were only 70 km from Paris and had occupied most of the small town, Chateau Thierry, at the northern bank of the Marne. On May 31st a cannonbattalion of the 3. US-Division, The Greens, took position on the southern bank and dug stands (?) for the guns (cannons). They had just had time to aim the 17 guns, when the Germans attempted to pass the river in a small, swift attack. The Yankees stopped the attempt. At the same time the 2. American Division (Marines and Infantry) blocked the road to Paris and beat off a German 3-day attack."
Peter wrote ".... and joined the Marines...." and "During four days of heavy artillery, he was hit by a German shrapnel....", so I think I'm close to something :). Further he died during the same period, and according to stuff I've found on the internet "Chateau Thierry" is a place the Americans remember from the Great War.
On the next pages of the book, it says:
The 4th Marine Brigade attacked on June 6th.......
First on June 25th the Bois Belleau was captured......
2. Division had 8000 dead and wounded, half of them from the 4. Marine Brigade.......
So, my guess is he was in the 4th Marine Brigade, and died during the battle of "Bois Belleau"/Marne. If you look at this map, the "Bois Belleau" is a few kms/miles left of Chateau Thierry, above "21" and right next to the motorway.