RIGA, Latvia — Exercising in Latvia’s dense pine and birch forests this week, local troops found themselves in an unfair fight against a new enemy: unmanned ground vehicles. As NATO tries to keep pace with fast-changing drone warfare, the alliance used Latvia’s Crystal Arrow exercise to test unmanned ground combat, equipping opposing forces with wheeled robots. The systems gave the red team an element of surprise over a blue team relying only on aerial drones, said Lt. Col. Andris Brūveris ,