Complex sentence: The Hỏa Lò Prison was a prison used by the French colonists in Vietnam for political prisoners, and later by North Vietnam for U.S. Prisoners of War during the Vietnam War when it was sarcastically known to American POWs as the "Hanoi Hilton".
Split into simpler sentences: The Hỏa Lò Prison was a prison used by the French colonists in Vietnam for political prisoners, and later by North Vietnam for U.S. Prisoners of War during the Vietnam War. During this later period it was sarcastically known to American POWs as the "Hanoi Hilton".