Complex sentence: Headquarters in the United States (Guardian's Office US) were in Los Angeles, California, though smaller offices existed in Washington, D.C. (Guardian's Office DC) and other cities throughout the United States.
Split into simpler sentences: Headquarters in the United States (Guardian's Office US) were in Los Angeles, California. A smaller office also existed in Washington, D.C. (Guardian's Office DC) and other cities throughout the United States.