Complex sentence: Until 1798 many ancient tiaras existed in the Vatican, but invading French troops stole and smashed all the tiaras they could get their hands on, stealing their jewels, which they transported back to Paris.
Split into simpler sentences: Until 1798 many ancient tiaras existed in the Vatican. However invading French troops stole and smashed all the tiaras they could get their hands on, stealing their jewels, which they transported back to Paris.