Complex sentence: British, Americans or other Europeans could not operate clandestinely in cities or populated areas in Asia, but once the resistance movements engaged in open rebellion, Allied armed forces personnel who knew the local languages and peoples became invaluable for liaison with conventional forces.
Split into simpler sentences: British, Americans or other Europeans could not operate clandestinely in cities or populated areas in Asia. However, once the resistance movements engaged in open rebellion, Allied armed forces personnel who knew the local languages and peoples became invaluable for liaison with conventional forces.