There were various WWI postcards issued to soldiers off to Europe, which were sent to the addressee back home once the transport ship had safley arrived, but giving no other information for security reasons. This is an envelope and letter serving that same purpose, which reads in part, "Am on the ship now it is great never saw anything like it, can't hardy wait until we get started. This is a German ship that the Americans captured at beginning of the war, will carry 5,000."
At the time I write this caption, the last surviving member of the American military who served in WWI died yesterday, February 28, 2011.