Very Good and Simple
I own this entire series from Hunted Cow for the iPad and find myself going back to it whenever I want a simple short game to spend some time with that requires very little brain power. The missions are fun and easy to learn. Each scenario is graded for 3 levels of difficulty: casual, normal, and hard, and you receive points based on how well you do for each level of difficulty. The series will remind you of the original board based war-games of the Avalon Hill days (if you are old enough to remember them) and are just as much fun. Highly recommended.