See CMoS 6.16–53
Commas are not necessary after short introductory adverbial phrases, particularly of time, unless needed for clarity. There is no definitive rule on ‘short’: two or three words is definitely short; six or more words is not.
In 1215 the great Mongol warrior Genghis Khan…
In the last 120 years of Manchu rule, Beijing and much of China were subject to invasion and rebellion.
When a city and state or city and country are listed in a sentence, a comma always surrounds the larger region - except in cases in which the larger region falls at the end of the sentence.
Taking the train from Seattle, Washington, to San Francisco, California, is the best way to see the West Coast.
Angkor Wat, one of the top tourist attractions in Southeast Asia, is located near Siem Reap, Cambodia.
LP deviates from the CMoS on serial/Oxford commas; they are not to be included unless needed for clarity.