I see no problem using the word "anniversary". Anniversary isn't necessarily positive or negative - it's neutral, indicating simply that something happened a certain number of years ago. I see it as often referring to things like national tragedies such as the 9/11/2001 attacks as I do for something celebratory such as a wedding.
I do agree, however that "Premium Quality Playing Cards" means as much to me as "The check is in the mail." Were they certified by some quality review board, or did someone's aunty say they were really nice? It's meaningless in this context.
I'd be surprised if you didn't include the flag of France on the new tuck - a large amount of the fighting took place in that nation. You're using the wrong US flag - the 35-star flag you depicted was taken out of commission on July 3, 1865; for the full duration of "The Great War", the flag had 48 stars on it, in a grid 8 stars wide and 6 stars tall.
The box back looks a bit bland as well. Perhaps you could depict flags of the US/UK/Fr alliance on the front, with the opposition on the back? Or maybe a map of Europe at the time war broke out as a wraparound?
Now, what do the cards look like - faces and backs? Who will be your printer?