Complex sentence: Two things of note occurred: first, the Vietnamese sent an army south to attack the Champa kingdom in 1446; second, the Dowager Empress ordered the execution of Trịnh Khả, for reasons lost to history, in the year 1451.
Split into simpler sentences: Two things of note occurred, first, the Vietnamese sent an army south to attack the Champa kingdom in 1446. Second, the Dowager Empress ordered the execution of Trịnh Khả, for reasons lost to history, in the year 1451.