What kind of challenges do the fearless veterans face as they train with elite military units in the documentary series “Toughest Forces on Earth”?

The fearless veterans face a variety of challenges as they train with elite military units in the documentary series "Toughest Forces on Earth." Some of these challenges include intense physical training, mental toughness tests, simulated combat scenarios, adapting to different environments and cultures, working under extreme pressure, and facing real-life dangerous situations such as taking down a narco laboratory in Colombia. These veterans must push themselves to their limits and beyond in order to keep up with the elite units and succeed in their missions.