Complex sentence: According to Marcellinus Comes, in 390 Arcadius expelled her from the Palace., but, since Arcadius was only thirteen, that decision could as well belong to those who governed in his name.
Split into simpler sentences: According to Marcellinus Comes, in 390 Arcadius expelled her from the palace. However, since Arcadius was only thirteen, that decision could as well have belonged to those who governed in his name.