Complex sentence: By 825 they had firmly established themselves in southern and southeastern Myanmar and founded the cities of Bago (Pegu) and Thaton, and by the mid- 9th century, they had come to dominate all of southern Myanmar.
Split into simpler sentences: By 825 they had firmly established themselves in southern and southeastern Myanmar and founded the cities of Bago (Pegu) and Thaton. By the mid- 9th century, they had come to dominate all of southern Myanmar.