Complex sentence: A Sino-Tibetan treaty was agreed upon in the year 821 A. D. Chinese missions arrived in Lhasa to meet with Tibetan generals to swear to an oath of peace between the two states.
Split into simpler sentences: A Sino-Tibetan treaty was agreed upon in 821. A Chinese missions came to Lhasa to meet with Tibetan generals and swore an oath of friendship between the two states.