Complex sentence: In the 19th century, the Bugis, Acehnese, and the Thai all attempted to invade Perak, and only British intervention in 1820 prevented Siam from annexing Perak.
Split into simpler sentences: In the 19th century, the Bugis, Acehnese, and the Thai all attempted to invade Perak. Only British intervention in 1820 prevented Siam from annexing Perak.