Complex sentence: The peak was at that time within the borders of British India, and in late 1939, with the start of World War II, Harrer was detained by British colonial authorities as an enemy alien, and interned at Dehradun, along with 1,000 other enemy aliens.
Split into simpler sentences: The peak was at that time within the borders of British India. In late 1939, after the start of World War II, Harrer was detained by British colonial authorities as an enemy alien, and interned at Dehradun, along with 1,000 other enemy aliens.