You know, songs about lives ruined by war, or the bitter kind of protest songs may not be appropriate for a "support the Troops" event.
"Last Night I Had the Strangest Dream" would be good, I think. How about soldiers' gripe songs? F'r instance
"I'll Tell You Where They Were",
"What Do You Do in the Infantry (You March, You March, You March)",
Ewan MacColl's "Browned Off,"
"Gee Ma I Want to Go Home ," or whatever.
Hard Crackers Come Again No More
Oh How I Hate to Get Up in the Morning
The Reuben James
Last Night I Had the Strangest Dream
"Come Be A Soldier for Marlboro and Me" (Tune: same as Waltzing Matilda)
Oh It's a Lovely War
Don't know if you know any of these or if they appeal to you but anyway maybe they'll suggest something.