Defence Expert Award
Throughout the past, some commanders have really gone above and beyond the call of duty. It's well known that attacking is much easier then defending. If you manage to hold the line for an average of 50 waves anytime after 10 missions, the U.S. Army will spread word that you are a master of defense, boosting anyone's morale under your command by 10.
Note that this medal can be received in any mission after your tenth, not just the ones whose game numbers are multiples of 10.